Definition of longshot
A longshot is a game that has very little chance of success. The term can be used in poker, but also in other disciplines. For example, if you go to the horse races and your horse has odds of 95:1 (95 to 1), you know it's a longshot bet. The bad news is that you won't win your bet very often, but the good news is that when you do win your bet, you will make a lot of money.
An example of a longshot in poker
Some positional players will like longshots. They probably figure they have an unlikely draw and if they hit, they'll make a lot of money and if they don't hit, they'll always have the option to bluff since they're in position. Let's say a player opens 67 in clubs on the button. The player in the small blind has AA and goes with a standard 3bet. The player in position calls with his hand. The button knows that the small blind is very strong since it doesn't play often. The flop comes T-2-K with a single club. The small blind bets and the button calls for a longshot. Here, we know that the small blind almost always has a big pair. The player on the button usually has a 10% chance of winning the bet on the showdown.