Short for "You're the man, bro. Check your pants." Said in a sarcastic tone, this indicates that the individual has either said something that makes absolutely no sense OR is conceited in nature. "You're the man, bro" is sarcastic, indicating that the person is the man, but only to himself. The "check your pants" part of the phrase tells said individual to check his manhood for being extremely ignorant and/or just a general prick.
Can also be used in short as a cordial greeting: i.e. 'checkem.
Fred: "Dude, I'm going to get so much ass tonight its ridiculous." Jeff: "Manbrocheckem."
Jerry: "Ohhhhh, I thought you said 'I bucked her in her mask'." Tony: "Manbrocheckem."
Sarah: "Heyyyy Sean! Whats up?" Sean: "'Checkem"