Comparative form of 'greener': more green in color, or more inexperienced and naive.
From 'green' with the comparative suffix '-er.' This double meaning (color and inexperience) exists because 'green' has been metaphorically linked to youth and naiveté since at least Shakespeare's time.
It's fascinating that the same word describes both a color and a personality trait—'he's green with envy' and 'he's a greenhorn' are both using 'green' to describe something immature or raw.
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