In the old days of speakeasies—those Prohibition-era nightclubs where so-called bathtub gin flowed—a would-be customer gained admission by showing up at the door and knocking. After a few moments, a small flap in the door would swing open and a tough, intimidating face would peer out. If the visitor was in the know, he would speak the name of some frequent patron of the place ("Joe sent me" was often enough), whereupon the bouncer inside would unlatch the door and let him in.