thwack

The verb is probably: * partly onomatopoeic, from the sound of something being beaten (compare whack); and partly derived from Late Middle English twakken, twake (“to hit (someone) with something; to pat; to stroke”), probably from Middle English thakken, thakke (“to dab; to pat; to stroke”) [and other forms] (whence thack (obsolete except Britain, dialectal)), from Old English þaccian (“to beat; to pat; to touch softly, stroke; to strike gently, clap, tap”), from Proto-West Germanic þakkōn, from Proto-Germanic þakwōną (“to pat; to tap; to touch”), from Proto-Indo-European teh₂g- (“to grasp with the hand; to touch”). Doublet of tangent. The noun and interjection are derived from the verb. Cognates * Latin tangō (“touch”) * Old Dutch þakolōn (“to stroke”) * Old Norse þykkr (“a blow, thump, thwack”) (Icelandic þjaka, þjökka (“to beat, thump, thwack”); Norwegian tjåka (“to strike, beat”))