Green’s Dictionary of Slang

(UK black) to challenge, to encounter; to meet.

1924		P. Marks Plastic Age 293: Four years of studying and lectures and examinations, and the first time he bucked up against a bit of life he was licked.
1984		Smiley Culture ‘Cockney Translation’ 🎵 Ca’ you never know when them might buck up a cockney.
1999		(con. 1979–80) A. Wheatle Brixton Rock (2004) 199: How comes since you buck up on Flynn’s girl you haven’t stopped chatting about her?
2018-20		T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 Buck up - meet (with), run into.