A marshpit is something that All time lows Alex gaskarth (lead singer) and Jack barakat (lead guitar) invented during a concert. They used the word 'Marshpit 'to describe a one man moshpit which was rather pointless.
Guy one: Hey was you at that atl concert the other night?
guy two: yeah it was amazing right?
guy one: yeah, did you see that idiot who was in a moshpit on his own?
guy two: yeah, pointless really. The band laughed at him and called it a marshpit!