c. 1200, "solemn engagement to devote oneself to a religious order or life;" by c. 1300 as "a solemn promise," usually to God or a saint, to do something or make a change, typically in expectation of reward; from Anglo-French and Old French voe (Modern French vœu), from Latin votum "a promise to a god, solemn pledge, dedication; that which is promised; a wish, desire, longing, prayer," noun use of neuter of votus, past participle of vovere "to promise solemnly, pledge, dedicate, vow."

This is reconstructed to be from PIE root *wegwh- "to speak solemnly, vow, preach" (source also of Sanskrit vaghat- "one who offers a sacrifice;" Greek eukhē "vow, wish," eukhomai "I pray").

Also "bind oneself" to chastity (early 14c.).

1640s, "pertaining to religious devotion;" see devotion + -al (1). The noun meaning "devotional composition" is recorded from 1650s. Related: Devotionally.

c. 1200, of persons, "yielding reverential devotion to God," especially in prayer, "pious, religious," from Old French devot "pious, devoted, assiduous" (Modern French dévot) and directly from Latin devotus "given up by vow, devoted" (source also of Spanish and Portuguese devoto), past participle of devovere "dedicate by vow" (see devotion). Of actions, "expressing devotion or piety," late 14c. Meaning "sincere, solemn" is from mid-15c. Related: Devoutly; devoutness.

active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.