"be in great plenty," early 14c., from Old French abonder "to abound, be abundant, come together in great numbers" (12c.), from Latin abundare "overflow, run over," from Latin ab "off, away from" (see ab-) + undare "rise in a wave," from unda "a wave," from PIE *unda-, nasalized form of root *wed- (1) "water; wet". Related: Abounded; abounding; abounder "one who has plenty or is wealthy" (1755).
English seems to always have used in the -ou- spelling, though in Middle English an unetymological h- sometimes was added. The vowel in Old French abonder, abondance is a continuation of a Merovingian Latin scribal use of -o- for classical Latin -u- to attempt to identify a sound that had evolved since classical times. In French eventually this sound came to be represented by -ou-. Compare French tour "tower," from Old French tor, from Latin turris; court (n.), from Old French cort, from Latin curtus; French outre from Latin ultra, etc. However -o- remained before a nasal (as nombre from numerus, monde from mundum, etc.).