Origin of thwackprobably echoic
transitive verbthwacked, thwack·ing, thwacks
Origin of thwackImitative.
(third-person singular simple present thwacks, present participle thwacking, simple past and past participle thwacked)
From a variant (influenced by whack) of Middle English thakken (“to stroke”), from Old English þaccian (“to touch gently, stroke, tap”), from Proto-Germanic *þakwōną (“to touch lightly”), from Proto-Indo-European *tag-, *taǵ- (“to touch”). Cognate with Old Dutch þakolōn (“to stroke”), Old Norse þykkr (“a thwack, thump, blow”), Icelandic þjökka, þjaka (“to thwack, thump, beat”), Norwegian tjåka (“to strike, beat”), Latin tangō (“touch”). More at tangent.