I believe the meaning to be the crossing of wee strains that go all over the toilet edge, or seat if you didn't put it up.
This was until recently limited to men, but I have discovered, much to the horror of you ladies out there, that Pissel does and can occur when you go too! Round the front of the toilet seat where it must deflect off the pan or something.
Boy: what? I can't help it, it's just my pissel
Girl: you've pissed all over the seat again!
Bloke: No I haven't.
Girl: There's piss all over the front of the seat?!
Bloke: That's you you silly girl, it's your pissel