A negative word used by shallow and inconsiderate people to describe themselves or others when they a) look stupid or b) act stupid, because they think looking and acting dumb is a trademark sign of Down's syndrome. Used by people who have never met someone who actually has Down's syndrome, and who are ignorant of and insensitive to a serious condition.
Cassie: Ewww! I look so downsy in this facebook picture, haha. I look sooo gross! I'm untagging myself!
Anne: Um, you look fine.
Rob: Yeah, shut the fuck up. My sister has Down's syndrome.