This term originates among Rastafari, and nobody knows if it is mash it up, mess it up, etc. For Rasta, moshing it up means destroying Babylon System which means paying respects and praises and learning from the tree of knoweldge. Young rastas mosh it up when they resist totalitarian police, when they demonstrate, and when they fight. For thrasmetal fans, moshing it up is destroying conformity and defying authority. Typically seen in pits in front of stage areas, moshing it up for thrashers is a way to demonstrate contempt for and agression to pressure. Typically involves slamdancing, skanking, agressive expressions of all types. Similarly, a thrashmetal musician can mosh it up when he/she is really good.

A person who doesn't realize they are transgender yet, or is in denial; an undeveloped transgender person. When they realize, it's referred to as the "egg cracking."

A term related to moshing at hardcore shows- usually stated (screamed) in the form of a command, at the start of a breakdown.
Those who mosh it up also frequently keep it real (for them and their friends.)