be a greenhorn

Primarily heard in US.

1. To be very young, naïve, immature, or inexperienced and, often, extremely gullible or easily deceived as well. When I first started here, I was a total greenhorn and actually believed those doofuses when they told me that the boss loves to have "Happy Birthday" sung to him in Spanish! This is her first job—she's a real greenhorn. Leave Julia alone. That sweet girl is a greenhorn and apt to fall for all your usual tricks.

2. To be a newcomer or newly arrived immigrant, especially one who is unfamiliar with the customs or ways of a place or group. Steve, Sally here is our newest greenhorn. Please show her 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! I was a real greenhorn when I first moved to France, but, luckily, I made some ex-pat friends, and they helped me immensely.