Thwack \Thwack\ (thw[a^]k), v. t. [imp. & p. p. {Thwacked}; p. pr. & vb. n. {Thwacking}.] [Cf. OE. thakken to stroke, AS. [thorn]accian, E. whack.] 1. To strike with something flat or heavy; to bang, or thrash: to thump. "A distant thwacking sound." --W. Irving. [1913 Webster] 2. To fill to overflow. [Obs.] --Stanyhurst. [1913 Webster]