Green’s Dictionary of Slang

13 n.

[the initial letter of Mexican is 13th in the alphabet]

(US prison) a symbol showing affiliation to a Mexican gang.

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[US]Mayor’s Anti-Gang Office City of Houston, TX Street Gang Sl. / Gloss. 9 Nov.: 13 or XIII or X3 or trece Thirteenth letter in the alphabet – (M), which symbolizes or identifies gang affiliation of Mexican heritage. Also may refer to allegiance to Southern California gangs.