An Etymological Dictionary of the German Language/tünchen
tünchen, vb., ‘to whitewash, plaster,’ from the equiv. MidHG. tünchen, OHG. tunihhôn, mostly mit chalche tunihhôn. The additional expression in OHG. leads to the assumption that tunihhôn means lit. ‘to clothe,’ from OHG. tunihha, ‘dress’ (borrowed, like AS. tunuce, from Lat. tunica). The Germans still say eine Wand mit Tünche bekleiden, ‘to give a coat of limewash to a wall’ (comp. E. to coat and coat). With this agrees Ital. intonicare, ‘to plaster, rouge’ (intonico, intonicato, ‘plaster’). Note that Lat. tunica, Ital. tonica, also signifies ‘covering.’ The Lat.-Ital. word was borrowed at the beginning of the OHG. period; had it been borrowed earlier it would have appeared as *zunihha in OHG. It cannot have been introduced much later than Fenster and Turm.