Antonyms of adj aforethought

1 sense of aforethought

Sense 1
aforethought (postnominal), planned, plotted -- (planned in advance; "with malice aforethought")

INDIRECT (VIA premeditated) -> unpremeditated -- (not premeditated)

Similarity of adj aforethought

1 sense of aforethought

Sense 1
aforethought (postnominal), planned, plotted -- (planned in advance; "with malice aforethought")
       => premeditated (vs. unpremeditated) -- (characterized by deliberate purpose and some degree of planning; "a premeditated crime")