From Longman Dictionary of Contemporary Englishgive somebody (a) free reingive somebody (a) free reinINDEPENDENT PERSONto give someone complete freedom to do a job in whatever way they choose → rein
Examples from the Corpus
give somebody (a) free rein• No, it was thanks to my culinary abilities that Marie-Claude gave me free rein of both her kitchen and her bedchamber.• Then I pretty much give them free rein.• I would discuss the script, say, on proportional representation, and then give him free rein.• While watching him at work she impulsively asked to borrow his materials and followed his advice to give her imagination free rein.• Such as he can not comprehend that which prompts others to give their heart free rein! 