Green’s Dictionary of Slang

1. to arrest (for debt).

1589		R. Harvey Pl. Perc. 11: Cap him sirra, if he pay it not [OED].
c.1607		Beaumont & Fletcher The Knight of the Burning Pestle III i: Therefore, gentle knight, Twelve shillings you must pay, or I must cap you.

2. (UK Und.) to take, to grab hold of.

1865

Leaves from Diary of Celebrated Burglar 67/2: I’m blarsted if thau’ll get me to ‘cap’ any moar ‘pokes;’ dost t’ ’ear? Here, thee ‘cap’ som on ’em look wat I’ve emptied and chuck ’em way.