Definition
This idiom advises against being critical or unappreciative of a gift or favor that someone has offered.
She gave me an old sweater, and even though it was a bit outdated, I accepted it graciously, remembering not to look a gift horse in the mouth.
When you receive something for free, remember not to look a gift horse in the mouth.
They offered us free lodging, and even though it wasn't a five-star hotel, we shouldn't complain; after all, you don't look a gift horse in the mouth.