Were you high when you got that idea? Because it's probably strange, unusual, weird, odd, absurd etc... not a thing a normal person would say.

It means I assume you were probably smoking some kind of drug, but your idea is so weird that it must have been a drug I don't know about. Or specifically, only a very strange drug could make someone come up with an idea as weird as that. Is this from the Simpsons? The "Poochie the Dog" episode? Otto: Whoa! A talking dog! What were you guys smoking when you came up with that one? Exec: We were eating rotisserie chicken.

Since smoking drugs (like weed) can really alter your thinking process/decision making process, a lot of people use "What were you smoking?" as a way of saying, "What was going on in your head when you made this decision?" It is a negative expression. It essentially means, "What the hell is wrong with you?"

Well dear there are many kind of smoking in our world...like weeds and grass, coking, etc... so these kind of question may occur that...what are you smoking...is you are smoking a cigarettes, or weeds, or grass, or something eles..

I guess it means ''Are you OK?''