Shorting = borrowing coins to sell them, in the hopes that the price per coin will be lower when you have to give them back. A "short sell" is to sell an asset (BTC) you don't own, you just have borrowed it to sell it because you anticipate the price to go down.

That's extremely dangerous with BTC, because the price has the potential to double multiple times very quickly, and thus you can face huge losses by shorting.

That's what "shorting" means, but in these forums many people say they are "short" when they have sold coins, which is not correct. What can you expect in a forum where most of people thinks that ROI is a synonym for break-even....