early 15c., lit. "in duty," from phrase put (oneself) in dever "make it one's duty" (a partial translation of O.Fr. mettre en deveir "put in duty"), from O.Fr. dever "duty," from L. debere "to owe" (see debt). One's endeavors meaning one's "utmost effort" is from late 15c.
Related: Endeavored; endeavoring.
British spelling of endeavor (q.v.); for suffix, see -or. Related: Endeavoured; endeavoring; endeavours.