let (someone) loose (on something)
To allow someone to do something as they please, without supervision or control. It's so nice to go to the park and let the kids loose for a while. I hope the board of directors aren't let loose on our project. We can't afford to have them changing things last minute!
let loose
(with something) Go to let go (with something).
let ˈloose
(British English) (American English cut ˈloose) (informal) do something or happen in a way that is not controlled: Teenagers need a place to let loose.