LATIN DICTIONARYLexicon Latīnum
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“promise” in Latin

The English word promise itself goes back to Latin mittō, through prōmissa.

Riddle & Arnold A Copious and Critical English–Latin Lexicon, 1864

promise s.
promissio; pollicitatio (Vid: the verb): fides (one’s word given or pledged to a certain effect): promissum (that which one has promised, a thing promised). To give a promise, alicui promittere or polliceri (☞ not alicui promissum facere); with an accusative and infinitive., de aliqua re: he makes firm promises (ironically), satis scite tibi promittit (in comedy): to make many promises, multa alicui polliceri: to keep or fulfil one’s promise, promissum facere, efficere, praestare, servare, solvere, exsolvere, persolvere; promisso stare or satisfacere; quod promisi or pollicitus sum, or quod promissum est, servare, observare, or efficere; quod promissum est tenere; fidem servare, praestare, solvere, or exsolvere: promises are not kept, promissa ad irritum cadunt: to be bound by a promise, promisso teneri.
promise v.
To make a promise, promittere (usually, in answer to a request, to engage one’s self to a performance of the thing required at some future time; alicui aliquid, or de aliqua re): polliceri (for the most part, spontaneously or of one’s own accord; also = not to refuse; alicui aliquid, or de aliqua re: ☞ neque minus ei prolixe de tua voluntate promisi, quam eram solitus de mea polliceri, Cic., ad Div., 7, 5, 1; promittere here denoting to give the actual expectation of a thing in prospect, polliceri merely to manifest a willingness to do it if possible): pollicitari (with repeated assurances): appromittere (to take upon one’s self a promise already made by another in one’s name): in se recipere, or simply recipere (to undertake, to become responsible for): recipere et ultro polliceri; spondere, despondere (formally, so as to come under a legal obligation to fulfil one’s engagement): proponere (to propose or hold out as a reward; e.g., servis libertatem): pronunciare (publicly). “To promise that or to,” is expressed in Latin by an accusative and infinitive (usually a future infinitive, rarely the present) after the verb of promising; e.g., I promise to do so or so, promitto, or polliceor, me hoc facturum esse: to promise in marriage, despondere (☞ not desponsare).
To give hope, promittere; spem facere or dare alicujus rei. He promises well, aliquis alios bene de se sperare jubet; alii de aliquo bene sperare possunt.

Quick answer

What is the Latin word for promise?
The usual Latin words for “promise” are prōmissiō, pollicitātiō, fidēs; also prōmittō, polliceor, efficiō. The right choice depends on the sense; see the distinctions below.

Entry from J. E. Riddle & T. K. Arnold, A Copious and Critical English–Latin Lexicon (rev. C. Anthon, New York: Harper, 1864), digitised by Fergus J. P. Walsh, CC BY-SA 4.0; Latin markup adapted by latindictionary.co.