The crossword solution for the term 'HE'S ON FIRST' is WHO, a three letter word. This phrase is a famous line from the comedy routine 'Who's on First?' by Abbott and Costello. The routine is a classic example of a double act in which one person misunderstands the other's words. In this case, Abbott is trying to explain to Costello the names of the players on a baseball team, but Costello keeps misunderstanding him.