Green’s Dictionary of Slang

aggressive, overbearing, domineering.

1883		J.T. Keane On Blue-Water 190: After that, no sailor will deny that a ‘bucko mate’ is not sometimes useful.
1899		H.E. Hamblen Yarns of Bucko Mate 51: lA committee of boarding-masters went the rounds, looking for volunteers to tame the Osceola’s bucko captain and officers.
1899–1900		C.L. Cullen Tales of the Ex-Tanks 370: The bucko first mate was wearing the boots.
1918		A.N. Depew Gunner Depew 18: There is a saying in the merchant marine that the bucko mate of a Lime-juicer is the toughest guy in the world, but they do not think so in the navy.
1939		K. Tennant Foveaux 212: Tommy enjoyed being first mate of a bucko ’bus; for those were the mad days of ’buses.
1941		G. Kersh They Die with Their Boots Clean 45: I do not doubt that even then, and there, there was a bucko sergeant who yelled, ‘Order yer arms when you got yer dressing!’.
1969		R. Jessup Sailor 299: You, the bucko mate [...] you hunkering around ashore mooning over some lost love!