c. 1200, devocioun, "profound religious emotion, awe, reverence," from Old French devocion "devotion, piety" and directly from Latin devotionem (nominative devotio), noun of action from past-participle stem of devovere "dedicate by a vow, sacrifice oneself, promise solemnly," from de "down, away" (see de-) + vovere "to vow" (see vow (n.)). From late 14c. as "an act of religious worship, a religious exercise" (now usually devotions).
In ancient Latin, "act of consecrating by a vow," also "loyalty, fealty, allegiance;" in Church Latin, "devotion to God, piety." The application to secular situations came to English via Italian and French; sense of "act of setting apart or consecrating" is from c. 1500.