green light

A traffic light in its green state (indicating that vehicles may proceed).

"The drive home was much faster than usual; somehow we managed to hit every green light."

Approval, or permission to proceed.

"We have a green light on the project. Work starts today."

A go-ahead to commit a crime, specifically to carry out a hit on someone (i.e. to kill).

"Yo man, I heard someone had put a green light out on you, so watch your back."