Don't look a gift horse in the mouth

Origin

For centuries, a horse's worth was judged by its teeth; a quick glance revealed its age and health. This scrutiny was essential for buyers but considered an insult when receiving a gift. To peer into a donated horse's mouth was to question the giver's generosity, implying you doubted its quality or were ungraciously appraising its value. This simple act of politeness, rooted in the ancient practice of horse valuation, evolved into a timeless reminder to accept gifts with gratitude, without suspicion or critique.