20 Surprising Origins Of Popular Sayings
The cat's out of the bag on these idioms.
1. "Let the cat out of the bag."
2. "Don't look a gift horse in the mouth."
Horses' gums recede with age, leading to longer teeth. A common way to inspect a horse's "worth" is to check it's mouth, hence the phrase. Receiving a horse as a gift and immediately inspecting its value was considered offensive, much like inquiring about the worth of a present today is rude.