Okay, so the '70 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
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."
An older horse's teeth would be ground down from all the chewing. If
the teeth were ground down, the animal would be less likely to be able
to ingest enough food. In the wild, for many older herbivores death comes
from malnutrition due to this inability to chew enough food. A person would
look in the horse's mouth to see the amount of wear, and the subsequent
Apparently, there are also several other things that happen. One is
that younger horses actually have more teeth. Another is that there is
a certain age range (like 6-10 years) where the teeth actually have some
circular bands. After this age range, the bands are worn away.
Hence you can tell a lot about a horse from inspection of the teeth.