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 *teh₂g- (“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. It should also be noted that early foreign scribes of Middle English confused "th" and "wh", as did some writers. This disappeared for the most part once Middle English spelling had developed.
thwack (plural thwacks)