Green’s Dictionary of Slang

brilliant (stark-naked) n.

raw, undiluted gin.

1819		‘One of the Fancy’ Tom Crib’s Memorial to Congress 35: As we’d been summon’d thus, to quaff / Our Deady o’er some State Affairs [...] Deady’s gin, otherwise, Deady’s brilliant stark naked.
1832		Egan Bk of Sports 62: I’ll bet you a kavarten of Seager’s brilliant that it is a bit of good truth.