Taken from the words bum and toucher. A bum toucher can be defined as someone who touches either alot of bums or perhaps one frequently. It can also be used to describe somebody who touches thier bum with other or perhaps their own.
They could also be known as an "ass feeler"- somebody who takes it upon themselves to feel others bums without their concent, which is frankly quite rude.
"Have you been touchin bum?eh eh cheeeky!"
"errgh you big bum toucher"