Don't look a gift horse in the mouth
Don't question the value of a gift. The proverb refers to the practice of evaluating the age of a horse by looking at its teeth. This practice is also the source of the expression “long in the tooth,” meaning old.
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.