Green’s Dictionary of Slang

(US) first-rate, out of the ordinary, notable.

1829		G. Lincecum in Hudson Humor of the Old Deep South (1936) 94: Fulahooma had privately engaged forty brag players and had given me their names.
1837		N.Y. Daily Express 22 June 2/5: [New York’s] fashionable boarding houses [...] are by no means equal to that of the first hotels, yet the expense of living in them (including brag bills) amounts to more than double.
1848		D.P. Thompson Locke Amsden 14: Thinking I would boil down a few pounds as nice as I could for brag-sugar [DA].
1904		W.N. Harben Georgians 291: Our brag murderer [...] recently laid low one of our most popular citizens.