Green’s Dictionary of Slang

bucko n.1

[buck n.1 + -o sfx (1), thence naut. jargon bucko, an overbearing ship’s officer, who enforces his will through (threats of) violence, usu. as bucko mate]

1. [late 19C+] a bully, a blustering swaggerer.

2. [late 19C+] a general term of address, e.g. my bucko.

3. [1910s+] a (spirited young) man.