The phrase originates from checking a horse's health by looking at its teeth, deemed impolite if the horse is a gift. Just enjoy the free meal and remember not to look a gift horse in the mouth—it's a kind gesture.
Pro Tip 2/3
Cultural Sensitivity
Some cultures may not be familiar with the phrase, so explaining its meaning could be helpful. When I said 'don't look a gift horse in the mouth,' my international friends didn't understand until I explained.
Pro Tip 3/3
Colloquial Usage
It's commonly used in casual conversation rather than formal writing or speech. He gave you a lift, so don't look a gift horse in the mouth by complaining about his driving.