ATDM is short for arse to different mouth.
it is a variation for the more commonly used term "ATM".
which is arse to mouth in porno's.
Basically it involves you shafting a birds arse, pulling out and putting your cock in her mates/sisters/mothers mouth.
Hence the abbreviation "ATDM"
i.e-- ive just done a bit of atdm, with that slag and her mate i met in tantra.