let it/her rip
(mainly American informal) if someone lets a vehicle rip, they make it move very fast She put her foot on the car's accelerator, and he said, 'OK, let her rip'.
let rip
1. to suddenly express your emotions without control This time I was furious and I let rip. He's a very restrained sort of person - you can't imagine him ever really letting rip.
2. (British & Australian very informal) to allow gas to escape from your bottom loudly You can't just let rip when you're in a smart restaurant.
See let rip