free rein

noun2

1. •
   A rein of a horse held loosely so that the animal can move more freely; also, the free condition that this gives to a horse.

   • I needed no whip, or spur, for I was as eager as my rider: he saw it, and giving me a free rein, and leaning a little forward, we dashed after them. — 1877
2. •
   (figurative) The absence of constraints; freedom to act, decide, or speak.

   • [H]e vvas pleas'd to lend / Free reines to mirth, and to suspend those cares / That claime such interest in th' Imperiall brovv. — 1645