An activity involving lots of peoples buttocks being blasted at once EG similar to a gang bang but its only arses being blasted.
Arseathon came from the word marathon as it is similar to a marathon only all of the people taking part in the activity are not running, they are taking part in arse blasting as a form of pleasure or just for the competition.
" man i walked into a pub the other night, i couldnt get out of there fast enough they were a right bunch of arseathon lovers"
"My friend went to this gay club the other night with a gay friend of his, apparently there was a right old fuckin arseathon going on in there "
"Hey darling do you fancy taking part in an arsethon session at mines later on?"
"sure ill bring the lube"