An Etymological Dictionary of the German Language/Annotated/einzig

by Friedrich Kluge

einzig, adjective, ‘only, sole, unique,’ from Middle High German einzec, ‘single,’ a developed form of Old High German einazzi (adverb, einazzêm), the zz of which is derivative, as in emsig (compare Greek κρυπτάδιος with a cognate suffix).