"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.
adjectival word-forming element, variant of -al (1) with connective -i-. From Latin -ialis, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective.
<a href="https://www.etymonline.com/word/conspiratorial">Etymology of conspiratorial by etymonline</a>
Harper, D. (n.d.). Etymology of conspiratorial. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/conspiratorial