Never look a gift horse in the mouth

[smurfchops]

Explain please ! I know what it means but where did this phrase come from?

[Big Al1st]

Never look a gift horse in the mouth is said in order to discourage too great an inspection of a gift, which might be less valuable than first thought, thereby taking the gilt off the gingerbread. To those who know about horses the analogy is clear; the age of a horse is well mirrored in the state of its teeth. Too close an inspection could result in a nasty surprise with the animal proving to be long in the tooth.

[brian j john]

http://www.trivia-library.com/b/origins-of-say ings-dont-look-a-gift-horse-in-the-mouth.htm