look a gift horse in the mouth means hat when someone gives you a gift, don’t go about finding fault with it. Accept it for what it’s worth.
Examples: My friend is leaving for the U.S for six months. She said that I could stay in her flat for free. Though there are several things wrong with the flat, I’ve decided not to look a gift horse in the mouth.
Origin: People can tell how old a horse is by looking at its teeth. The longer the teeth, the older the horse is. So when someone gifts you a horse, don’t open its mouth to find out how young or old it is. If it is old, don’t complain about it. After all, the horse is a gift. Accept the gift for what it’s worth.