Kendall - from Wikipedia (below). "Moshing is rough, physical dancing and the area in front of that kind of stage is the "mosh pit". Not exactly ceilidh stuff though!

The term mosh came into use in the early 1980s American hardcore punk scene in Washington, D.C. Early on, the dance was frequently spelled mash in fanzines and record liner notes, but pronounced mosh, as in the 1982 song "Total Mash" by the D.C.-based hardcore band Scream. H.R. of the band Bad Brains, regarded as a band that "put moshing on the map," used the term mash in lyrics and in concert stage banter to both incite and to describe the aggressive and often violent dancing of the scene. To "mash it up" was to go wild with the frenzy of the music. Due to his Jamaican-accented pronunciation of the word, fans heard this as mosh instead.

By the 1980s, the term was appearing in print with its current spelling. By the time thrash metal band Anthrax used the term in their song "Caught in a Mosh", the word was already a mainstay of hardcore and thrash scenes. Through the mainstream success of bands like Anthrax, Nirvana and The Melvins, the term came into the popular vernacular.