vow (n.)

"solemn promise," c. 1300, 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," from PIE root *wegwh- "to speak solemnly, vow, preach" (source also of Sanskrit vaghat- "one who offers a sacrifice;" Greek eukhe "vow, wish," eukhomai "I pray"). Meaning "solemn engagement to devote oneself to a religious order or life" is from c. 1400; earlier "to bind oneself" to chastity (early 14c.).

"promise solemnly," c. 1300, from Old French voer, from voe (see vow (n.)). Related: Vowed; vowing.