When a person that is supposed to know what he's doing spaces out and does the complete office. The term was originally coined to refer to former Red Sox baseball player: Manny Ramirez. It can also be used in different context to refer to any idiot.
Sam: Look at that fool that just got hit in the head with a baseball.
Jim: Manny being Manny
Britney being Britney (slut)
Paris being Paris (bigger slut)
Lindsy being Lindsay (idiot) (and slut)
Alex being Alex (eternal failure to win a world series)