Definition: An Arnie is a side bet that is won by a golfer who makes par on a hole without ever being in the fairway. The amount of the bet is set before the round begins. Arnies are not something a golfer sets out to win, however - the round is played with the intent of playing as well as possible (no sandbagging, in other words). However, if along the way a golfer makes par on a hole without hitting the fairway, the Arnie is his reward.
The bet is named for Arnold Palmer, who made quite a few pars in his career on holes where he failed to find the fairway.