Emily Dickinson Lexicon

THWACK, v.t. [Qu. Sax. thaccian, to feel or stroke lightly. It does not well accord with this verb. The word twit is the Sax. æthwitan, or othwitan, a compound of æth or oth, to or at, and witan. In like manner, thwack may be formed from our vulgar whack, which is precisely the Eth. ወቅዐ wakea, Ar. wakaa, to strike.]

To strike with something flat or heavy; to bang; to beat or thrash. Arbuthnot.