This movie is stupid.
This movie is gay.
This movie is retarded.
This movie is dumb.
This movie is lame.
What's the difference between these five?
They're all insults... They're all used in the same contexts, but literally:
stupid: not smart, unintelligent
retarded: suffering from mental retardation, i.e. medically stupid
dumb: can mean stupid or mute, but when used as an insult it typically means stupid
lame: literally means you can't walk, but in this context it implies boring
My sister is constantly using the word gay as a substitute to the word weird.
Note that some people might find "gay" in this context offensive (perhaps because of the implication that being "gay", i.e., homosexual, is a bad thing).
A lot of people find "retarded" in this context offensive as well, and it's a pretty offensive word in general, because it's perceived as an insult to the mentally handicapped. Amusingly, it was originally a euphemism.