Never look a gift horse in the mouth 英文諺語
Never look a gift horse in the mouth 餽贈之物切莫挑剔
To be ungrateful to someone who gives you something; you look in a horse’s mouth to determine its age and/or health. Since horses’ teeth grow over time, checking their length is a way of gauging old age. So if someone gives you a horse, and you look the gift horse in the mouth, it’s like looking for a price tag. It’s rude!