Green’s Dictionary of Slang

metzel n.

[Ger. Metzelsuppe, metzel soup, made with sausage]

(US) a German-born immigrant.

[US]Maledicta VII 21: German-Americans were called metzel deriving, I speculate, from Metzelsuppe or metzel soup (made with sausage), a popular dish in the 19th-century immigrant communities.