1. n. A fictional creature from the Harry Potter series. Dementors guard the wizard prison Azkaban and suck all of the happiness from anything in their presence. Their worst weapon is called the dementor's kiss, which is when a dementor sucks a person's soul out through their mouth.

2. n. A person who kills a good moment or sucks all of the fun out of a situation.

Someone who is in the fringe of your group of friends who is tolerated because it would be a shame to tell them to Fuck off!
They also have some strange ability to turn even the funniest momements into akward ones and sap all happiness from those around.

Living depression

1. A really really hot chick. She's so hot she's scary. It can also be a really really hot guy, but it mostly for women. This dementor makes something silvery come out of your wand.

Woman's menstrual period. Goes along with Blast-ended Skrewts (cramps) and Azkaban (uterus).
It sucks.

The most Ballin Drink you can make, Brass Monkey a 40 of Old English with Purple Drink, called the Dementor because it resembles "Dementor Piss" according to Jonny Haworth. WARNING: you will feel like you just got the kiss of death from a Dementor if you can conquer this beast.

An apathetic person. If you stare for too long she/he will steal your soul.