A pro-wrestler who is so unable to perform basic actions correctly and at the right time, that they almost certainly are unable to wipe their arses properly, leading to them walking around with their arses being all shitty. Hence, shitarse.
Man, I just watched a match involving noted shitarse Magnus. He's fucking terrible, at everything.
A portmanteau of the English swear “shit” and “Squidward”, the name of a character on SpongeBob SquarePants. Typically interjected in situations of surprise or shock, however can also be used as an insult.