It’s hard to escape it these days. It is sweeping the world like an epidemic. When you see someone’s fingers and thumb start to curl into their palm and they begin to raise their arm, there’s only one thing that could be coming… the fist pump or ‘guts pose’ as the Japanese call it. (source)
Read more about the actual history of the fist pump here.
Really the fist pump probably doesn't qualify as an idiom but it is a part of our speech that needs to be explained from time to time so I decided to include it. (My blog my choice! *wink*)