(Verb) Originating from Yorkshire, this is a slang term for the act of masturbating, or "bashing". To bash one out, means to have a good masturbating session.
Steve left the room midway through the film to bash one out. I heard him moaning in the next room.
A: Just saw a guy in the park bashin' his Dachshun'
B: What like physically abusing a defenseless little wiener?
A: Not little but yeah absolutely mandhandling the poor sausage.
B: You should've got involved...
A: Nah not my cup of tea.