twelvemonth n. + Add to list UK //twˈɛlvɪmˌʌnθ// twelve·month Archaic SynonymsEtymology Synonyms yearyrOriginOEtwelfmōnþ→MEtwelmonth→ENtwelvemonth From Middle English twelmonth, twelfmonthe, twelfmonþe, from Old English twelfmōnþ, twelfmōnaþ, equivalent to twelve + month.