300K means 300 Killers. 300 is a click of the Black Disciples located in the South Side of Chicago. Rival gang members of 300 often claim they are 300K. Adding a K at the end of a rival gang’s name is very common in Chicago, for example lot gang members claim the following, 600k, BDK, GDK. In Killa Kellz’ “300K” song, he said “300 killa I'm killin 300 n*ggas, fuck a 300 n*gga Ion' fuck with 300 n*ggas, b*tch”