Why is the ninth month called September?
1590s, from Latin instigator, agent noun from instigare (see instigation). Fem. formation instigatrix is recorded from 1610s.
1540s, back-formation from instigation or else from Latin instigatus, past participle of instigare "to urge on, incite" (see instigation). Related: Instigated; instigates; instigating.