From Middle English, from Old English swīþ (“strong, mighty, powerful, active, severe, violent”), from Proto-Germanic *swinþaz (“strong”), from Proto-Indo-European *swento- (“active, healthy”). Cognate with Old Saxon swīth, Middle High German swind (Modern German geschwind (“fast, quick, swift”)), Dutch gezwind (“fast, quick, swift”), Old Norse svinnr, Gothic (swinþs, “strong”). Related to sound.
(comparative swither, superlative swithest)
From Middle English swith, swithe, from Old English swīþe (“very much, exceedingly, severely, violently, fiercely”), from Proto-Germanic *swinþa (“strongly”), from Proto-Indo-European *swento- (“active, healthy”). Not cognate to superficially similar swift, as these have distinct PIE roots, though both share *swe- prefix in PIE.