Merriam-Webster Collegiate Dictionary - bimbo

 noun  (plural ~s)  Etymology: perhaps from Italian ~ baby  Date: 1918  1. slang man, woman telling a thickheaded pitcher that the ~ at the plate hasn't hit a curve in three seasons — Jay Stuller — used as a generalized term of disapproval especially for an attractive but vacuous person we didn't want a blond ~ in that role…we wanted her to be smart — Hugh Wilson  2. slang tramp 1c evidence of how her hubby's been cheating on her with various ~s — Dan Greenburg