From Middle Dutchvucht, vocht. Substantivized form of an identical adjective, from Old Dutch*fuhti(“humid, wet, damp; marsh-like”), from Proto-Germanic*funhtijaz(“humid, wet, damp”). In Middle Dutch the word gradually shifted from being an adjective to being a noun, with the old adjective being replaced by vochtig(“humid, wet”).
Cognate with Old English fuht(“moist, damp”) and German feucht(“wet, humid, damp”).