dog whistle

Meanings

1. A high-pitched whistle, generally inaudible to humans, used to train dogs.
2. That which is understood only by a narrow demographic.
3. A political allusion or comment that only a certain audience is intended to note and recognize the significance of; particularly in relation to controversial or extreme viewpoints.

Etymology

dog + whistle. The figurative senses derive from the whistle's sound frequency, which is too high to be heard by human but can be heard by dogs.