An idea or insight a person has when they are high. For the duration of being high, it is a truly profound, original, and useful idea, but turns out the next day to make no sense, or be of any use whatsover.
Jimbo had an highdea last night that he was sure would make him a multi-millionalre. The next morning, however, he realized that no one would be interested in purchasing live ducks to wear as headwear.
"damn man, I wish that when you dropped your bong, it would just break into several smaller bongs. Someone should invent that."
"If i had a sandwich for every one of your highdeas, I would have entirely too many sandwiches."
at the time, this idea seems like it has dawned upon you by some divine intervention of the gods. it can either be a bad idea or a good idea, but you think it's the greatest thing that has ever happened.
I had a highdea last night to get in the hottub, it was the greatest thing ever; I felt like a blow up doll floating in the water.