A "slock" (lock in a sock) is an improvised weapon sometimes used by inmates. The design is simple; a padlock is inserted into a sock. The open end of the sock is gripped tightly and swung at a target, typically another inmate, with the intention of causing physical injury.
A slock was used by Vee in Season Two to attack Red in the greenhouse Red had established to smuggle in items to Litchfield inmates. Vee had previously approached Red offering a truce, but it was revealed she'd lied. She found Red in the greenhouse and struck her with the slock several times, injuring her severely ("It Was the Change").
Vee later blamed Suzanne Warren for the attack, gaslighting Suzanne into believing that maybe she was responsible by having used Suzanne's padlock, nicknamed Lady Loxley, for the attack ("We Have Manners. We're Polite.").