A "March Up", often used in ye olde England in a sentence "March Up The Town" ...meaning to either protest en masse against the political system, or, later to engage in hooliganism of many kinds....got appropriated and Patois-ized into "MASH up de town" by the Caribbean blacks who were emigrating and traveling back and forth to the UK, and that's how "gon' mash up de town" got into the lexicon of reggae/ska lyrics...and they pretty much used it in the same way, protest, gang action, general going out en masse with a posse to effect some sort of group "excitement" (Brixton, Islington, etc) which might also include some dancing...the proto punks of the UK as we already know (like the Clash etc.) were huge fans of reggae and ska, and appropriated it in it's shortened form "mash", in some cases maintaining it's meaning of political or social protest, but that bled over into the new mass-dancing and energy of the punk-rock movement.. but the English meets Cockney meets West Indian meets Jamaican accent certainly sounds way more like "mosh", and that's the way the Cali and NY punk kids in their respective language-challenged ears heard and re-appropriated it for further redefining as a way to describe THEIR en-masse energetic actions, specifically, in the Pit (remember when it used to be SLAM DANCING? I didn't think so). But in doing so, historically-revised it into something never or rarely thought of as BLACK in origin (indeed, I bet most of the denizens of the world of the modern American MOSH PIT would be loathe to incorporate little if any Africanism into their mostly white pursuit)... especially white-supremacist Skinhead types, though they might, in their ass-backwards way of "white-i-fying" everything, hearken back to the days of yore and say that the blacks who came to England were the one's who really modified it's original meaning. I wonder if in their urge to be ultra-correct, in a WHITE way, would start calling a Mosh pit a MARCH PIT! Unfortunately, most interesting slang that has been kicking around and morphing for centuries, comes to a sudden and boring, dumb-ass demise at the hands of young, white Americans.
"We's gonna march up the town, TOMMY"
"Mi g'wan mash up de town wit me mon WINSTON"
"DEREK...it's a mash up...oi!"
"isn't that JASON in the mosh pit?"