Look a gift horse in the mouth
Definition 1 of 1
Pro Tip 1/3Origin Reference
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 2/3Cultural 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 3/3Colloquial 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.