To turn something off or make it stop working; to make something inactive or non-functional.
From 'de-' (prefix meaning removal or reversal) plus 'activate' (from Latin 'activus' meaning 'doing' or 'active'). This is a modern compound that became common in the 20th century with technology and chemistry.
The prefix 'de-' is incredibly powerful in English—you can take almost any verb and add it to create an opposite meaning ('activate/deactivate,' 'code/decode,' 'construct/deconstruct')—which is why English speakers can understand brand-new technical terms they've never heard before!
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