Allowing entry or access; acknowledging or confessing something to be true.
From Latin 'admittere' meaning 'to allow to go to' or 'to give access'. The sense of 'confessing' developed because allowing something into one's mind or speech was seen as giving it access to reality.
The phrase 'admitting defeat' reveals the spatial metaphor underlying this word - we imagine defeat as something outside ourselves that we either keep out or allow in. This same metaphor appears in 'admitting evidence' in legal contexts.
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