Having been taught and practiced a skill or behavior until becoming proficient at it.
From Old French 'trainer' meaning 'to drag or pull.' The idea evolved to mean 'to guide in a certain direction,' which naturally extended to teaching someone through guided practice.
The word 'trained' literally started as 'to drag' but became about teaching because you guide someone along a path toward knowledge—it's a metaphor so old we forget it's a metaphor anymore.
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