As this is the urban dictionary,
I suppose the best definition would be 'sticking something up your arse'! More specifically and for those of a more 'technically minded' nature, it is the use of suppositories or pessaries (hence killing at least one myth, that the 'pessary' is a
small bird that lives in New Zealand!..that's a
cassowary).
Balanism doesn't usually have anything to do with masturbation or
marsupials, contrary to popular
misconception.