Originally defined as 'Half way between a kiss and a blowjob"
You know the mouth movement one does when performing a blowie?
It's like that, except on the lips. Sucking someones lips instead of properly kissing. Often done when you're trying to make out with someone for the first time, although if the other person is doing it first time too, I guess you're Ok.
"Is he any good at making out?"
"Fuck no, he just gave me a sloppery kissjob..."