Broken down, "pissed-up" means inebriated or drunk. A "slapper" is, well, a slut. Or, a woman of loose morals. So, a "pissed up slapper" is also known as a drunken trollop or whore.
"Lets go to the bar tonight and pick up a couple of pissed up slappers." "Check that pissed up slapper going down on Mark in the corner." "It's a sure thing cause she's a pissed up slapper."
Came across it c.50years ago when a teenager on a building site.
I think it may be where "pissed up" meaning to be drunk came from.
Foreman: "It's Monday and he asked for a sub, he got paid on Friday! All his wages pissed up the wall"
Me: "What's pissed up the wall?"
Foreman: "What? Oh, it's going out and getting drunk."
Me: ....?
Foreman: "Staggering home they'll feel like a piss and go against the nearest wall: what they'd spent their wages on just gets pissed up the wall."