look a gift horse in the mouth

Idioms  

1. Be critical or suspicious of something received at no cost. For example, Dad's old car is full of dents, but we shouldn't look a gift horse in the mouth. This term, generally expressed as a cautionary proverb (Don't look a gift horse in the mouth), has been traced to the writings of the 4th-century cleric, St. Jerome, and has appeared in English since about 1500. It alludes to determining the age of a horse by looking at its teeth.

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Example Sentences

He told the Daily Telegraph in 2009, after criticism of the deal: "It's not my business, the second the price is paid. If that is the offer, I'm not going to look a gift horse in the mouth and suggest they have overpaid me."

From BBC

Never look a gift horse in the mouth, goes the old saying.

From Barron's

Trumble pointed to the famous expression "don't look a gift horse in the mouth" as proof that people have always suspected a connection between health and examining an animal's teeth.

From Salon

“You can’t look a gift horse in the mouth,” said Morrow, a retired teacher.

From Seattle Times

But whatever the reason, best to not look a gift horse in the mouth.

From Slate