Permission to go ahead, as in The chief gave us the green light for starting this project. This term originated in the late 1800s for the signal used by railroads to indicate that a train could proceed. It was transferred to more general use in the first half of the 1900s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.