Running off at the mouth is an occupational hazard of a DJ's job and when you're Superjock Larry Lajack of WCFL Chicago, all dolled up in your Levis, boots and flowered cowboy shirt, a dozen screeching teenyboppers begging for your autograph, you can afford to be a snide, smart-assed s.o.b. In fact, you get paid for it. Lujack specializes in insulting people on and off the air; the ""Klunk Letter of the Day"" is a regular feature of his show--between the screams of David Bowie and the Clearasil spot. Lujack has a lot of boys-in-the-locker-room one-liners about the chicks who come to the station to ball their favorite radio ""personality"" right in the Djs' lounge (he claims he never indulges) and the others who send their phone numbers and vital statistics. Several-""DJ of the Year"" plaques line his walls but in case you overlook them he's not shy about announcing: ""I'm the best damn rock 'n' roll disc jockey that's ever lived."" Even though you might wonder when he tells you that ""I could never understand the appeal of Janis Joplin"" who was ""the absolute snottiest person I ever met."" Maybe she just had taste.