To open something locked with a key or mechanism, or to reveal or discover something hidden or secret.
From Old English 'un-' (reversing prefix) plus 'loc' or 'lock' from Proto-Germanic. The metaphorical meaning of revealing secrets developed over time.
The metaphor of 'unlocking' knowledge or secrets is so natural that many ancient philosophers described learning as unlocking the mind—as if education is just revealing what was always there! Modern apps and games now literally use unlock mechanics to pace discovery.
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