Green’s Dictionary of Slang

(US) to carry a gun; to arm oneself.

1999		J. Ridley Everybody Smokes in Hell 203: The goddam liberals get in the way of our basic freedom to strap up.
2004		Simon & Alvarez ‘Homecoming’ Wire ser. 3 ep. 6 [TV script] Yo fellas. Strap up.
2012		‘Dutch’ ? (Pronounced Que) [ebook] ‘Hell no, I ain’t scared.’ ‘Then strap up,’ YaYa told him.
2020		D. Winslow ‘Paradise’ in Broken 250: If the Palala wants to strap up [...] I’m on board.