definition of thwack
â¢ THWACK (noun). The noun THWACK has 1 sense: 1. a hard blow with a flat object. Familiarity information: THWACK used as a noun is very rare. â¢ THWACK (verb). The verb THWACK has 1 sense: 1. deliver a hard blow to. Familiarity information: THWACK used as a verb is very rare.