1535-45; < Latininstīgātus past participle of instīgāre to goad on, impel, equivalent to in-in-2 + -stīg- goad, prick (akin to stigma, stick2) + -ātus-ate1
1540s, back-formation from instigation or else from Latin instigatus, past participle of instigare "to urge on, incite" (see instigation). Related: Instigated; instigates; instigating.