late 14c., countrevaillen, "to be worth as much as," also "to prevail against, resist with equal force," from Anglo-French countrevaloir, Old French contrevaloir "to be effective against, be comparable to," from Latin phrase contra valere "to be worth against" (see contra- and valiant). Related: Countervailing.
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."
early 13c., prouesse, "an act of bravery;" c. 1300, "military bravery combined with skill in combat," from Old French proece "prowess, courage, brave deed" (Modern French prouesse), from prou, later variant of prud "brave, valiant," from Vulgar Latin *prodem (source also of Spanish proeza, Italian prodezza; see proud (adj.)). Prow was in Middle English as a noun meaning "advantage, profit," also as a related adjective ("valiant, brave"), but it has become obsolete. "In 15-17th c. often a monosyllable" [OED].
From mid-14c. in English as "having beneficial or efficacious properties;" late 14c. (of persons) as "having excellent moral qualities; conforming to religious law." Related: Virtuously; virtuousness.