Green’s Dictionary of Slang

(US prison) any form of bludgeon-like weapon, e.g. batteries or a padlock in a sock.

1990		Kool G Rap ‘’Rikers Island 🎵 ’Cause in every cellblock there is a hard rock / With a nice device called a sock lock.
2002		(con. 1998–2000) J. Lerner You Got Nothing Coming 199: I heard about you in the Fish Tank — making a killer slock out of a newspaper!