- abound (v.)
- "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" (see ab-) + undare "rise in a wave," from unda "a wave," from PIE *unda-, nasalized form of root *wed- (1) "water, wet" (see water (n.1)). 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 excrescent 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.).