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."
2 people, mainly a dude and a chix, pressedtight against one another in a way that their hotspots (genitals) are makin' contact. This usually happens during intimate kissing and hugging. It can be standing up or sitting or laying down but is usually with clothes still on. Not dryhumping, just as it says, "Pressed up"
dude: damn we got all pressed up 2day, that wuz hawt
chix: god i wanted to grind u today
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."