An Etymological Dictionary of the German Language/Annotated/munter
munter, adjective, ‘cheerful, lively, brisk,’ from Middle High German munter, munder, Old High German muntar, adjective, ‘fresh, lively, zealous, wakeful’; probably allied to Gothic mundrel, feminine, ‘aim,’ and mundôn, verb, ‘to fix one's eyes upon,’ so that ‘striving’ is the originally meaning of the adjective. It is also primitively allied to Old Slovenian mądrŭ, ‘wise,’ Lithuanian mundrùs, mandrùs, ‘cheerful.’ Moreover, Old High German muntar may also be connected with Old High German męnden, Old Saxon męndian, ‘to rejoice’ (Teutonic root manþ).