To be very young, immature, or inexperienced. Are you sure he's fit to run your campaign? He's very green—I mean, he only graduated from college six months ago. Please don't accept any applications from candidates who are too green—I need an experienced executive secretary, not someone who still needs their diapers changed.
See also: green
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.