postman's knock
noun
1
a lock. Rhyming slang, ascribed to burglars when used by lockmakers Chubb's in an advertisement UK, 1962.
2
a clock. Rhyming slang, sometimes seen in an abbreviated form as 'postman's' UK, 1992.
3
in pool, a shot in which the cue ball hits the object ball twice in rapid succession, producing a knock-like sound US, 1993