So THAT'S what that means. #3
Never look a gift horse in the mouth.
By looking in a horses mouth you can see the quality of its teeth, and figure out the general health of the horse. Also you can determine an approximate age.
If someone gives you a horse as a gift, looking in its mouth would imply that you are inspecting it to see if it is worthy. Which would also be pretty rude.
If you get a gift, be thankful no matter what it is. Never look a gift horse in the mouth.