To die by jumping in front of an oncoming train. Comes from a combination of New York City subway code for "person under train" (12-9) and shorthand for "yourself" ("12-9 yourself"). Suggestion of how a person "ought to be killed" when one is annoyed with the other person. Similar in meaning to DIAF and DIAT. In other words: "shut up".
"Shut up, DIAF, and 12-9ys, moron. You have flamed enough for the both of us!"