this is when you are making out with your significant other and you casually put your hands on their cheeks, but then quickly plug their nose and try to suck the air out of their lungs.
While kissing french or otherwise, one partner will suck the air (soul) out of the other partners mouth and lungs. Also effective in sucking the happiness out of the room.
Steve: If I know I'm not getting any with a girl, I'll give her a Dementor's Kiss.
Dave: What?
Steve: I'll kiss her so I can suck all the air out of her and kill the mood even more.
In the Harry Potter series there is a creature called a dementor that is used to guard the wizarding prison of Azkaban. Their special power is the ability to take the joy away from anyone they are near and their most lethal attack is the "dementor's kiss" where they can suck the soul out of a person.
This term can also refer to a case where a poor unsuspecting dude in a bar locks lips with the ugliest broad there. This can either be done by force or with the aid of beer goggles. The result is that he feels miserable and disgusted with himself. Quite like having one's soul sucked out.
"Dude! Dude! I saw thatold hag give you the dementor's kiss. You ok?"
"duuuude I'm so ugh so *hiccup* wasted I couldn't even defwend meself."
During a make-out session with my boyfriend, I (quite sweetly) uttered the line “you take my breath away”. He leaned in to kiss me again, and as we were doing so he literally sucked the air out of my lungs before exclaiming “That’s what it feels like to be a Dementor!”
The worst thing…his geekiness actually turned me on a bit.
~ A Prisoner of Azkaban, College Humor, after experiencing a Dementor's Kiss