Is it farther or further?
early 15c., "pains taken to attain an object," literally "in duty," from phrase put (oneself) in dever "make it one's duty" (a partial translation of Old French mettre en deveir "put in duty"), from Old French dever "duty," from Latin debere "to owe" (see debt). One's endeavors meaning one's "utmost effort" is from late 15c.
c.1400; see endeavor (n.). Related: Endeavored; endeavoring.