Similar to power nap, the concept of taking a quick nap to relieve exhaustion, but applied to showering. One takes a power shower when they have just engaged in a sweaty activity (i.e., sex or sports) and doesn't have time for a full-length shower. Usually involved rinsing down with soap, instead of shampoo, conditioner, and shaving.
"I had a quickie with my boyfriend during my lunch break today!"
"Didn't you feel gross afterwards?"
"Well, I didn't have time to take the shower I wanted to, so I just took a power shower!"