Green’s Dictionary of Slang

buff adj.¹

naked.

1672		J. Phillips Maronides (1678) V 80: T’other lay’s on cuff after cuff, / Not minding whither’s skin be buff.
1707		N. Ward London Terraefilius III 23: A great Number of Flogging Instruments, for the Secret Flogellation [sic] of Superanuated Sodomites ty’d with Ribbons of all sorts of Colours [...] so that every Beast, from the Buff-Hided Leacher, to the Lamb-Skinn’d Cully understood, by the Colour, how to call for a Scourge that was most agreeable.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
2021		J. Ellroy Widespread Panic 76: Stretch snatched the buff shot and studied it [...] Oooh-la-la.

SE in slang uses

In compounds

buff-coat (n.) [the buff-coloured uniform; later use SE]

a soldier.

1632		T. Nabbes Covent Garden III ii: I know hee dare as well take the wall of a drunken Constable, or justle a Buffe-coat leading a wench, as meet me.
1680		E. Hickeringill Curse Ye Menoz 27: A Robber’s a Robber, though he Rob in a Fryar’s weed or a Buff-coat [i.e. as a soldier].
1692		J. Hacket Memorial of John Williams Pt 1 170: Some profane Buff-Coats will Authorize such Incendiaries.
c.1698		B.E. Dict. Canting Crew.

In phrases

buff to one’s work (v.) [coll. buff, naked]

to strip off preparatory to starting a fist-fight.

1823

Blackwood’s Mag. in DSUE (1984).