An Etymological Dictionary of the German Language/munter
munter, adj., ‘cheerful, lively, brisk,’ from MidHG. munter, munder, OHG. muntar, adj., ‘fresh, lively, zealous, wakeful’; probably allied to Goth. mundrel, f., ‘aim,’ and mundôn, vb., ‘to fix one's eyes upon,’ so that ‘striving’ is the orig. meaning of the adj. It is also primit. allied to OSlov. mądrŭ, ‘wise,’ Lith. mundrùs, mandrùs, ‘cheerful.’ Moreover, OHG. muntar may also be connected with OHG. męnden, OSax. męndian, ‘to rejoice’ (Teut. root manþ).