What's the origin of "Don't look a gift horse in the mouth"?
Dear Straight Dope:
Could you please tell where the term "Don't look a gift horse in the mouth" comes from and what you would have been trying to convey with this expression?
SDStaff Ken replies:
You're getting this straight from the horse's mouth.
Any decent breeder could tell you that one of the best ways to judge a horse is to look in its mouth. By counting its teeth, noting the extent to which the gums have receded, etc., you can get an idea of the horses's age and physical condition. However, if you got the horse as a gift--equivalent to being given a car nowadays--checking its teeth would suggest you're secretly thinking, "I'll be lucky if this nag lives until St. Swithin's Day." You simply say "thanks" and save all recriminations until you get home. This same practice gives us the now obsolete phrase "long in the tooth," a nice way of saying "old."