vigor (n.) Look up vigor at Dictionary.com
c. 1300, "physical strength, energy in an activity," from Anglo-French vigour, Old French vigor "force, strength" (Modern French vigueur), from Latin vigorem (nominative vigor) "liveliness, activity, force," from vigere "be lively, flourish, thrive," from PIE root *weg- (2) "be lively or active" (see wake (v.)).