Okay, so the '72 Gremlin grandma gave you is not your idea of a babe magnet, but it was free - so don't look a gift horse in the mouth. Horses have gum lines that recede with age. Hence older horses have longer teeth than young horses. To "look a horse in the mouth" is to examine the horse's mouth closely to determine its age (and therefore its usefulness and/or worth). To immediately judge a gift based on its worth or usefulness rather than the "thought" behind it considered rude, and ungrateful (it is a gift after all, and didn't cost the receiver anything). The phrase is apparently quite old, a Latin version of it appeared in a work by St. Jerome in 420 AD, and it also exists in many languages. An Early english version (1510 AD) appears in John Standbridge's "Vulgari Standbrigi": "A gyuen hors may not (be) loked in the tethe."