An Etymological Dictionary of the German Language/einzig

by Friedrich Kluge

einzig, adj., ‘only, sole, unique,’ from MidHG. einzec, ‘single,’ a developed form of OHG. einazzi (adv., einazzêm), the zz of which is deriv., as in emsig (comp. Gr. κρυπτάδιος with a cognate suffix).