Top Definition
Just about the greatest comedy routine ever, by comedians Abbot and Costello, with Abott telling Costello the names of the players.

In this context, the first baseman is named "Who"; thus, the utterance "Who's on first" is ambiguous between the question ("which person is the first baseman?") and the answer ("Who is the name of the first baseman.").
"Costello: Whats the guy's name on first base?
Abott:What is the guy's name on second base
Costello: Im not asking you Who's on second!
Abott: Who's on first!
Costello: I dont know!
Abott: He's on third, we are not talking about him."

"That Who's on first? Routine always makes me lol"
by AJ Molina October 19, 2008

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the most basic of facts (from Who's on First?, a confusing comedy routine that was done by Bud Abbott and Lou Costello)
Apparently, most politicians don't even know who's on first about citizenship.
by The Return of Light Joker April 07, 2009

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