Green’s Dictionary of Slang

1. Mexican.

1853		G.D. Brewerton Overland With Kit Carson (1930) 154: The United States Hotel upon the plaza provided ‘chicken fixin’s and corn doin’s’ or, if a stranger wanted ‘Mex livin’,’ frijoles and tortillas to boot [DA].
1906		C. M’Govern Sarjint Larry an’ Frinds 29: De prices range from 100 pesos, for a beauty [...] to 50 cents Mex for an auld woman.
1926		H. Moore ‘The Lingo of the Mining Camp’ in AS II:2 88: In recent years in the copper camps farther south, a large Mexican element has been brought in. Of course, the American soon moves on, deploring the fact that the town has become a Mex camp.
1939		W. Attaway Let Me Breathe Thunder (1940) 20: Ain’t nothing there but Spick and Mex shacks.
1949		F. Brown Dead Ringer 98: I heard him start explaining how he was tying in the Mex turnover gimmick.
1957		Kerouac On The Road (1972) 89: Groups of Mex chicks swaggered around in slacks.
1963		in T.I. Rubin Sweet Daddy 22: Crooked as Mex two buck billtwo-doolarr.
1977		(con. 1911) J. Monaghan Schoolboy, Cowboy, Mexican Spy 131: There’s no law – Mex’ ner ’Merican – in them bastards camp.
1988		(con. 1940s) C. Bram Hold Tight (1990) 228: ‘No handouts. Get lost,’ said a mex-looking man.
1992		G.P. Pelecanos Firing Offense 128: ‘It was a Mex place or Spanish. That’s all I can remember’.
2000		F.X. Toole Rope Burns 208: We been too nice a you, protectin you mex ass and not chargin you.
2021		J. Ellroy Widespread Panic 15: A Mex busboy [...] saw my roscoe and got the shiver-shakes.

2. (US) second-rate, inferior; in measurement, half the length/weight/value, etc.

1899		New Oxford Item (Gettysburg, PA) 7/2: A Mexican dollar is accepted in trade as worth just half of an American dollar, so ‘Mex’ has come to mean half [...] A Nebraska volunteer [...] inquired how much further it was out to the firing line. ‘Eight miles,’ was the reply. ‘American or Mex?’ he eagerly inquired.
1906		C. M’Govern Sarjint Larry an’ Frinds 58: It was not annie ‘Mex.’ foive moile that we hiked.