countervail v.

word-forming element meaning "against, in opposition," from Latin adverb and preposition contra "against" (see contra (prep., adv.)). The Latin word was used as a prefix in Late Latin. In French, it became contre- and passed into English as counter-. The Old English equivalent was wiðer (surviving in withers and widdershins), from wið "with, against."

early 14c. (late 12c. in surnames), "brave, courageous, intrepid in danger," from Anglo-French vaylant, and Old French vaillant "stalwart, brave," present-participle adjective from valoir "be worthy," originally "be strong," from Latin valere "be strong, be well, be worth, have power, be able, be in health," from PIE root *wal- "to be strong." As a noun, "valiant person," from c. 1600. Related: Valiantly.

Proto-Indo-European root meaning "to be strong."

It forms all or part of: ambivalence; Arnold; avail; bivalent; convalesce; countervail; Donald; equivalent; evaluation; Gerald; Harold; invalid (adj.1) "not strong, infirm;" invalid (adj.2) "of no legal force;" Isold; multivalent; polyvalent; prevalent; prevail; Reynold; Ronald; valediction; valence; Valerie; valetudinarian; valiance; valiant; valid; valor; value; Vladimir; Walter; wield.

It is the hypothetical source of/evidence for its existence is provided by: Latin valere "be strong, be well, be worth;" Old Church Slavonic vlasti "to rule over;" Lithuanian valdyti "to have power;" Celtic *walos- "ruler," Old Irish flaith "dominion," Welsh gallu "to be able;" Old English wealdan "to rule," Old High German -walt, -wald "power" (in personal names), Old Norse valdr "ruler."