Unlike a solitary shank (shower and wank combined), a lamb shank is done in the presence of a sheep. Whilst this is traditionally a male lamb, at a pinch any ewe will do!
I was feeling a bit lonely last night so I had a lamb shank instead.
Another definition for having a wank.
'I caught Fred watching a porno the other night, I'm not 100% sure but I think he was in the middle of a lamb shank'.