To secretly change or damage something, usually to make it work differently or to hide something wrong.
From Dutch 'tampen' meaning to stop up or plug, which evolved into the meaning of meddling or interfering with something. The connection is about stopping up or blocking normal function.
The word 'tamper' is actually related to 'tampon'—both come from the idea of stopping something up—so when you tamper with evidence, you're technically 'plugging' it with false information.
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