Green’s Dictionary of Slang

a swaggering braggart.

1590		Three Lords and Three Ladies of London C: What a brag boy is this to comparison with a man, but boy [...] I will lay six Ballades to six ierkes at your buttocks, that you shall not sing so well as I.
c.1698		B.E. Dict. Canting Crew n.p.: Brag, braggadoeio [sic] A vapouring, Swaggering, Bullying Fellow.
1725		New Canting Dict.
1741		Braggadochio, or, His Worship, the Cully [play title].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: braggadacio a vain glorious fellow, a boaster.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.