Green’s Dictionary of Slang

1. a boaster, a braggart.

c.1570		Legend Bishop St Andrews n.p.: Proude ambitious bangsters [F&H].
1651		D. Calderwood Hist. Kirk II 516: My lord, mak us quite of thir Matchiavelian and bangester lords [F&H].
1768		A. Ross Helenore in Wattie Scot. Works (1938) 129: ’Tis now come till our hand what Bydby tauld: / We’ll naething be afore yon bangsters bauld.

2. a bully.

1594		Acts James VI 217: Persones wrangly intrusing themselves in the rowmes and possessiones utheris be bangstrie and force [F&H].
1768		A. Ross Helenore (1778) 169: Bangster, a bragger, or bully.