Green’s Dictionary of Slang

(US) a derog. term for a French, Spanish, Portuguese or Italian person; thus adj. garlic-eating.

1865		in R. Mitchell Civil War Soldiers (1989) 203: In May 1865 Lt. Christopher Keller anticipated that the next job that would face the U.S. army was fighting the French, whom he termed ‘frog-eaters’ and ‘garlic-eaters,’ in Mexico.
1878		Leeds Mercury 1 Nov. 6/2: Yes, that’s where a Spanish devil of a count ran me through with a small sword. Every time I catch a cold it settles in my lungs where the damned garlic-eater’s toad-sticker went through.
1884		Nat. Police Gaz. (NY) 5 Jan. 3/3: Maurice [...] speaks all the dialects of Europe, and is correspondingly esteemed by the garlic eaters.
1905		Sun. Times (Perth) 29 Jan. 1/1: Three garlic-eating grisettes have opened an establishment in a fashionable East Perth quarter.
1915		S. Ford Shorty McCabe on the Job 204: Heim gagen. Mushong! Gangway, gangway! [...] Ah, beat it, you garlic destroyers!
1957		Laurents & Sondheim West Side Story II iv: baby john: Gold tooth! diesel: Pierced ear! a-rab: Garlic mouth! action: Spic! Lyin’ Spic!
1971		I. Faust Willy Remembers 206: Now free of garlics thanks to the 6th and 16th.
1995		M.M. Gillan Where I Come From 82: On Columbus Day, he could forget the laughter of the Americans who spit at him on the street, called him ‘Dago, Guinea, Wop, Gangster, Garlic Eater.’.
2005		J. Danna Sicilian Project 55: My brother and I were fair game for mean kids who called us names like Ginny, Greaseball and Garlic Eater.