"one who plots or acts on evil or unlawful designs," c. 1400, conspiratour, from Old French conspirateur, from Latin conspiratorem (nominative conspiratorio), noun of action from past-participle stem of conspirare "to agree, unite, plot," literally "to breathe together" (see conspire).
Conspirer is attested from 1530s, from Anglo-French conspirour. Fem. form conspiratrice is from early 15c.; conspiratress is from 1760. Related: Conspiratory.