Old English dysig "foolish, stupid," from Proto-Germanic *dusijaz (cf. Low German düsig "dizzy," Dutch duizelen "to be dizzy," Old High German dusig "foolish," German Tor "fool," Old English dwæs, Dutch dwaas "foolish"), perhaps from PIE *dheu- (1) "dust, vapor, smoke; to rise in a cloud" (and related notions of "defective perception or wits").
Meaning "having a whirling sensation" is from mid-14c.; that of "giddy" is from c.1500 and seems to merge the two earlier meanings. Used of the "foolish virgins" in early translations of Matthew xxv; used especially of blondes since 1870s. Related: Dizzily.