To plan secretly with others to do something illegal or harmful; to work together secretly for a common (usually bad) purpose.
From Latin 'conspirare' (to breathe together), composed of 'con-' (together) and 'spirare' (to breathe). The image is of people huddling together whispering their plans.
The root 'spirare' (to breathe) also gives us 'spirit,' 'respiration,' and 'inspiration'—all about air and life force. So when conspirators work together, they're literally 'breathing together,' which is why conspiracy was often shown as shadowy figures huddled close, whispering.
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