Definition of "red light"

red light

noun
plural red lights
1. A warning light, especially as a traffic signal indicating stop.
2. A sign of a brothel.
  Quotations
  Roxanne, you don't have to put on your red light.
  1978, Sting (lyrics and music), “Roxanne”
3. (idiomatic) Denial to proceed; a ruling out of any possibility.