Sentence:  In his teens <WSD>O</WSD> ' Banion was enrolled in the vicious Market Street gang and he became a singing waiter in McGovern's Cafe , a notoriously low and rowdy dive in North Clark Street , where befuddled customers were methodically looted of their money by the singing waiters before being thrown out . 
Meaning of the word marked with <WSD> tags:  a human being