A person who is new to something and lacks experience or skill; a beginner or novice.
From 'green' (meaning young or inexperienced, from Old English) plus 'horn' (originally referring to a horned animal that is young). Used since the 1600s.
In the Old West, a 'greenhorn' was someone whose horns hadn't hardened yet—a metaphor for immaturity that captures something important about how experience literally hardens you against difficulty.
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