1. A very young, naïve, immature, or inexperienced person, especially one who is extremely gullible or easily deceived. Primarily heard in US. We all convinced that greenhorn in work that the boss loves to have "Happy Birthday" sung to him in Spanish!

2. A newcomer or newly arrived immigrant, especially one who is unfamiliar with the customs or ways of a place or group. Primarily heard in US. Alright, Steve, show the greenhorn around the ranch so she'll be ready for work tomorrow. It was tough when I moved to Iceland. I was such a greenhorn that I didn't even know the polite way to order coffee!