noun: (onomatopoeia) The sound of repeated tapping, knocking, or clicking.
noun: (countable) A device used to tap on a window or door from a distance.
noun: (UK, uncountable) Alternative form of tic-tac, a sign language used by bookmakers. [(Britain) a system of sign language used by bookmakers at racecourses to communicate odds and bets; an associated group of words for each sign]
