Old High German
An old compound mezzi- + sahs, cognate with Old English meteseax, metsax, from Proto-Germanic *matisahsą (“food-knife”). The Gothic equivalent would be (unattested) *matisahs, compare to attested 𐌼𐌰𐍄𐌹𐌱𐌰𐌻𐌲𐍃 (matibalgs, “food-bag”). In origin the knife used for cutting food while working, hunting or travelling, which doubled as a defensive weapon.
The compound was obscured at an early time, within the Old High German period, which suggests frequent use. The original mezzi-sahs is first reduced to mazsahs (Old Saxon mezas, Low German metz, mess, mest). The form containing -r- then appears still in the Old High German period, as mezzirahs which via mezzarehs, mezziras, mezzeres becomes mezzer. Middle High German has mezzer exclusively (with the exception of a 12th-century Rhenish mez-sehs).