>Just guessing ... but it's probably a contraction of "Can't be bothered getting off (or moving) one's arse". I believe arse is generally used to refer to the buttocks as well as the ET ot IT.

This sounds about right. It is not a term one would use in front of one's grandmother but is more like slang than an expletive. To be correct the ET ot IT would be more likely used in the much coarser expression "he is such an arsehole".