To mosh is to dance in a wild, almost violent way. Fans of music like hardcore punk and thrash metal often mosh at live shows.
Concert-goers first began to mosh in the early 1980s while listening to bands play loud, aggressive hardcore and punk music. A "mosh pit" is the area near the stage where fans dance crazily. It became more common to mosh when grunge gained popularity in the late 80s. This verb was first spelled mash, but pronounced mosh — probably thanks to the lead singer of the band Bad Brains, who encouraged crowds to "mash it up" in his Jamaican accent.