It refers to best gang the SURENOS{
SOUTHSIDERS}SUR means south and the 13 refers to the 13th letter in the dictionaryM for their original name Mexican Mafia aka
LA EME & since most of their member were from
south california they called each other Sureno{southerner}
Sureno:what U is
pussy ass Norteno:Nor 14
Sureno:sur 13 puto{unloads gun clip}
pussy ass Norteno:187ed{dead}