The sense "kind of footman's armour" is apparently a back-formation from almain rivet, which is apparently derived from the English noun; see that entry for more.
1550, Edward Hall, “(please specify the part of the work)”, in The Vnion of the Two Noble and Illustre Famelies of Lancastre & Yorke, Beyng Long in Continuall Discension for the Croune of this Noble Realme,[…], London:[…]Rychard Grafton, […] [and Steven Mierdman], →OCLC:
over his rivet he had a garment of white cloth of gold with a redde crosse
1903, The Archaeological Journal, page 105:
In 1579 it is mentioned that Almain rivets are now out of use, and in lieu of them a corselett shall be found. The rivets varied in cost; in 1509 they were to be had for 8s., in 1512 they were imported at 16s., and again in 1513 […]
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