"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.).
"copious quantity or supply," mid-14c., from Old French abondance and directly from Latin abundantia "fullness, plenty," abstract noun from abundant-, stem of abundans "overflowing, full," present participle of abundare "to overflow" (see abound).
early 15c., surrounden, "to flood, overflow," from Anglo-French surounder, Old French soronder, suronder "to overflow, abound; surpass, dominate," from Late Latin superundare "overflow," from Latin super "over" (see super-) + undare "to flow in waves," from unda "wave" (from PIE root *wed- (1) "water; wet;" compare abound). Sense of "to shut in on all sides" first recorded 1610s, influenced by figurative meaning in French of "dominate," and by sound association with round, which also influenced the spelling of the English word from 17c. Related: Surrounded; surrounding.
It forms all or part of: abound; anhydrous; carbohydrate; clepsydra; dropsy; hydra; hydrangea; hydrant; hydrargyrum; hydrate; hydraulic; hydro-; hydrogen; hydrophobia; hydrous; Hydrus; inundate; inundation; kirsch-wasser; nutria; otter; redound; redundant; surround; undine; undulant; undulate; undulation; vodka; wash; water (n.1); wet; whiskey; winter.
It is the hypothetical source of/evidence for its existence is provided by: Hittite watar, Sanskrit udrah, Greek hydor, Old Church Slavonic and Russian voda, Lithuanian vanduo, Old Prussian wundan, Gaelic uisge "water;" Latin unda "wave;" Old English wæter, Old High German wazzar, Gothic wato "water."
late 14c., redounden, "to overflow, flow abundantly; abound, multiply, increase" (senses now obsolete), also "to flow or go back" (to a place or person), "be sent, rolled, or driven back," from Old French redonder "overflow, abound, be in profusion" (12c.), from Latin redundare "to overflow" (see redundant). Hence "to contribute, have effect" (to the credit, honor, etc.), early 15c. Related: Redounded; redounding.