Etymology
The noun is probably from middle (noun) + -ing; the adjective is most likely derived from the noun, and the adverb from the adjective.
Adjective
middling (comparative more middling, superlative most middling)
- Of intermediate or average size, position, or quality; mediocre.
The football team is never the worst or best in its league; its position is always middling.
1700 February, Thomas Molyneux, “II. An Essay Concerning Giants. Occasioned by Some Further Remarks on the Large Humane Os Frontis, or Forehead-bone, Mentioned in the Philosophical Transactions of February 16845, Number 168.”, in Philosophical Transactions. Giving Some Account of the Present Undertakings, Studies and Labours of the Ingenious, in Many Considerable Parts of the World, volume XXII, number 261, London: Printed by S. Smith and B. Walford, printers to the Royal Society, at the Prince's Arms in St. Paul's Church-yard, published 1702, →OCLC, page 501:[…] I cannot think it unreaſonable, […] to imagin the ſame natural cauſes may ſometimes act in t'other extreme likewiſe, and model Humane Bodies from a peculiar Energy in the ſeminal Principles, or a more perfect and through concoction in the Stomach, and other Viſcera, whence may proceed a peculiar and extraordinary nutritive faculty in the humours, for the furthering augmentation; or ſtill from ſome other more latent Spring, or ſecret Influence, to ariſe to ſuch a growth as fully to equal twice the heighth of (what we may then properly call) a middling ſtature, taking the word in the moſt ſtrict ſenſe.
1783 July, “Foreign Literature. France. Art. XIV.”, in [Ralph Griffiths], editor, The Monthly Review; Or, Literary Journal, volume LXIX, London: Printed for R[alph] Griffiths; and sold by T[homas] Becket, No. 82, Pall Mall, →OCLC, page 61:Had it [De le Maniere d'écrire l'Histoire (Concerning the Manner of Writing History, 1783) by Gabriel Bonnot de Mably] appeared ſooner, and been peruſed by with docility by ſome of our minor adventurers in this walk of literature, it might have had a good effect, by ſhewing them, that it is not ſo eaſy a matter as they think to write hiſtory, though the moſt middling genius may fling facts, dawb characters, and copy tales and truths promiſcuouſly.
1853 January 1, “Parallel Passages”, in E[liakim] Littell, editor, Littell’s Living Age, volume XXXVI, number 450, Boston, Mass.: E. Littell & Son, →OCLC, page 29, column 2:All this is undoubtedly compatible with mediocrity, like every other profession; one can also be a middling poet, a middling orator, a middling author; but this done with genius is sublime.
1946, V[ictor] S[awdon] Pritchett, “Clarissa”, in The Living Novel, London: Chatto & Windus, →OCLC; republished New York, N.Y.: Reynal & Hitchcock, 1947, →OCLC, page 24:Yet there he [Samuel Richardson] is, plump, prosaic, the most middling of middling men, and so domestically fussy that even his gift of weeping hardly guarantees that he will be a major figure.
2012, Miriam L. Wallace, A. A. Markley, “Introduction”, in Miriam L. Wallane, A. A. Markley, editors, Re-viewing Thomas Holcroft, 1745–1809: Essays on His Works and Life (British Literature in Context in the Long Eighteenth Century), Farnham, Surrey, Burlington, Vt.: Ashgate Publishing, →ISBN; republished Abingdon, Oxon., New York, N.Y.: Routledge, 2016, →ISBN, page 5:Rather, [Thomas] Holcroft uses his experiences as fooder while claiming the status of an "author" just as more middling writers did.
2012 July 12, Sam Adams, “Ice Age: Continental Drift”, in The A.V. Club, archived from the original on 25 March 2014:The matter of whether the world needs a fourth Ice Age movie pales beside the question of why there were three before it, but [Ice Age:] Continental Drift feels less like an extension of a theatrical franchise than an episode of a middling TV cartoon, lolling around on territory that's already been settled.
- (colloquial, regional British) In fairly good health.
1728 September 18, [Jonathan] Swift, “[Letters on Several Occasions.] Letter XXIII. To the Same [Dr. Thomas Sheridan].”, in Miscellanies, 4th edition, volume XIII, London: Printed for R[obert] Dodsley in Pall-mall, published 1751, →OCLC, page 125:I am in a middling Way, between Healthy and Sick, hardly ever without a little Giddineſs or Deafneſs, and ſometimes both: So much for that.
1913, D[avid] H[erbert] Lawrence, “Strife in Love”, in Sons and Lovers, London: Duckworth & Co. […], →OCLC, part II, page 200:“And how’s that chest of yours?” demanded Mrs. Morel. / He smiled again, with his blue eyes rather sunny. / “Oh, it’s very middlin’,” he said.
Adverb
middling (comparative more middling, superlative most middling)
- (colloquial, regional British) Fairly, moderately, somewhat.
1785, Joseph Strutt, “HENRY GOLTZIUS”, in A Biographical Dictionary; Containing an Historical Account of All the Engravers, from the Earliest Period of the Art of Engraving to the Present Time; and a Short List of Their Most Esteemed Works. [...], volume I, London: Printed by J. Davis, for Robert Faulder, New Bond Street, →OCLC, page 343:St. Jerom ſeated, a middling ſized upright plate, from J. Palma, dated 1596. I think this is one of the fineſt prints by this great maſter. The drawing is admirable, and the engraving is executed with the utmoſt freedom.
- 1811, Engelbert Kempfer [i.e., Engelbert Kaempfer]; J[ohann] G[aspar] Scheuchzer, transl., “The Division and Sub-division of the Empire of Japan into Its Several Provinces; as also of Its Revenue and Government”, in The History of Japan; republished in John Pinkerton, editor, A General Collection of the Best and Most Interesting Voyages and Travels in All Parts of the World; Many of which are Now First Translated into English. Digested on a New Plan, volume VII, London: Printed for Longman, Hurst, Rees, Orme, and Brown, Paternoster-Row; and Cadell and Davies, in the Strand, →OCLC, page 665:
- Iwami, otherwise Sekisju, is two days journey long, going from ſouth to north, a middling good country, producing plenty of cannib, and affording ſome ſalt.
- (colloquial, regional British) Not too badly, with modest success.
Noun
middling (plural middlings)
- Something of intermediate or average size, position, or quality.
2014, Annachiara Longoni, “Organisational Responsibility and Worker Commitment: The Definition and Implementation of Sustainable Operations Strategies”, in Sustainable Operations Strategies: The Impact of Human Resource Management and Organisational Practices on the Triple Bottom Line (Springer Briefs in Applied Sciences and Technology; PoliMI SpringerBriefs), Cham, Switzerland: Springer, Springer Science+Business Media, →DOI, →ISBN, →ISSN, page 21:Middlings are less likely to adopt worker commitment than are Leaders, but the two groups are equally likely to adopt organisational responsibility. […] Middlings have not been able to effectively implement innovative environmental programmes and proactive social programmes.
- (in the plural) Preceded by the: people of moderate means; members of the middle class.
2014, Ed White, “The Shays Rebellion in Literary History”, in Andrew Lawson, editor, Class and the Making of American Literature: Created Unequal (Routledge Interdisciplinary Perspectives on Literature; 24), New York, N.Y., London: Routledge, →ISBN, page 70:The war had been significantly financed by the poor and the middling, who purchased bonds that quickly devalued from wartime inflation; debt speculators had bought much of this debt at a fraction of the original cost.