Green’s Dictionary of Slang

1. (US) a cowboy.

1936		P.A. Rollins Cowboy 76: The cowboy was frequently called ‘baquero,’ ‘buckeroo,’ [...] ‘boyero,’ and each subject to be contracted into ‘bucker’ [W&F].
1972		D. Dalby ‘African Element in Amer. Eng.’ in Kochman Rappin’ and Stylin’ Out 178: buckaroo, bucker—‘cowboy’.

2. (US) a toady, one who curries favour for self advancement.

1960, 1975

Wentworth & Flexner DAS.