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mid-14c., from Anglo-French conspiracie, Old French conspiracie "conspiracy, plot," from Latin conspirationem (nominative conspiratio) "agreement, union, unanimity," noun of action from conspirare (see conspire); earlier in same sense was conspiration (early 14c.), from French conspiration (13c.), from Latin conspirationem. An Old English word for it was facengecwis. As a term in law, from 1863. Conspiracy theory is from 1909.