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A widely popular phrase of abbreviated words to sound almost nothing like the original meaning:

1) A large (Massive) group gathering (Session) of people with the Y chromosome (Peen); Hanging out; Chillin with the bros.

2) Mass Peen Sesh can also refer either to Gay/homosexual intercourse between many men.
Dude, friday we're having a Mass Peen Sesh at joe's for some mad ish.

Mass Peen Sesh at Cinema 10 on Saturday, you comin' bro?

"Okay so likkkeeee were havvingg some mass peen sesh at my houseeeee ;) come with ur toyzzzz!"
by Phoocakez January 26, 2010
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