An Etymological Dictionary of Astronomy and Astrophysics

1a) A declaration that something will or will not be done, given, etc., by one.

1b) An express assurance on which expectation is to be based.

2. To engage or undertake by promise; to make a promise (Dictionary.com).

Etymology (EN): M.E. promis(se), from O.Fr. promesse “promise, guarantee, assurance” and directly from L. promissum “a promise,” from promittere “send forth; let go; assure beforehand, promise,” from → pro- + mittere “to release; send.”

Etymology (PE): Fargot, literally “word (say, saying) put forth,” from far-, variant of farâ-, → pro-, + got, from gotan, gutan (as in Kurd. (Kurm.) gôtin, guhtin, (Sor.) gutin, Gilaki gutan, Shahmerzadi. -got- “to speak”), variant of goftan “to say, tell, utter,” Mid.Pers. guftan; O.Pers. gaub- “to say.”