Unknown, perhaps dialectal. It seems to have originally referred to a lucky shot at billiards. Possibly connected to sense 3, referring to whales' use of flukes to move rapidly.
“[…] That's the first time in the history of Bierce's Cove that two men made that jump on the same sea. And all the risk was yours, coming last.” “It was a fluke,” Billy insisted.
1920, Zane Grey, “The Rube”, in The Redheaded Outfield:
Three of the best hitters in the Eastern retired on nine strikes! That was no fluke.
1930, Dorothy L. Sayers, Strong Poison:
"And I say," said Wimsey, "that it would be better for her to be hanged outright than to live and have everybody think her a murderess who got off by a fluke."
Mid-16th century in the sense of “anchor blade”. Probably the same word as in etymology 2 above or else a related word for something flat (cf. Proto-Germanic*flakaz). A derivation from Middle Low Germanvlögel(“wing”), from Proto-Germanic*flugilaz, seems phonetically impossible. If anything, related vlōch, vlucht(“flight”, both also “wing”) or even *vlunke (modern Low GermanFlunk(“wing, pinion”)) are more plausible candidates. Note that the kind of whale's fin is called Fluke in contemporary German, but this is likely from English.
(nautical) Any of the triangular blades at the end of an anchor, designed to catch the ground.
The fluke of the anchor was wedged between two outcroppings of rock and could not be dislodged.
1904–1906, Joseph Conrad, chapter IV, in The Mirror of the Sea, 1st American edition, New York, N.Y., London:Harper & Brothers, published October 1906, →OCLC:
The honest, rough piece of iron, so simple in appearance, has more parts than the human body has limbs: the ring, the stock, the crown, the flukes, the palms, the shank. All this, according to the journalist, is “cast” when a ship arriving at an anchorage is brought up.
Either of the two lobes of a whale's or similar creature's tail.
The dolphin had an open wound on the left fluke of its tail where the propeller had injured it.
But though this sculpture is half man and half whale, so as only to give the tail of the latter, yet that small section of him is all wrong. It looks more like the tapering tail of an anaconda, than the broad palms of the true whale's majestic flukes.
As Walter de la Mare writes, "How uncomprehendingly must an angel from heaven smile on a poor human sitting engrossed in a romance: angled upon his hams, motionless in his chair, spectacles on nose, his two feet as close together as the flukes of a merman's tail, only his strange eyes stirring in his time-worn face."
A metal hook on the head of certain staffweapons (such as a bill), made in various forms depending on function, whether used for grappling or to penetrate armour when swung at an opponent.
The polearm had a wide, sharpened fluke attached to the central point.
In general, a winglike formation on a central piece.
After casting the bronze statue, we filed down the flukes and spurs from the molding process.
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James Orchard Halliwell (1846) “FLUKE”, in A Dictionary of Archaic and Provincial Words, Obsolete Phrases, Proverbs, and Ancient Customs, from the Fourteenth Century. [...] In Two Volumes, volumes I (A–I), London:John Russell Smith,[…], →OCLC, page 365, column 2.