What does "look a gift horse in the mouth" mean?

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.