Source : Webster's Revised Unabridged Dictionary (1913)

Aforethought \A*fore"thought`\, a.
   Premeditated; prepense; previously in mind; designed; as,
   malice aforethought, which is required to constitute murder.
   --Bouvier.

Aforethought \A*fore"thought`\, n.
   Premeditation.

Source : WordNet®

aforethought
     adj : planned in advance; "with malice aforethought" [syn: {aforethought(ip)},
            {planned}, {plotted}]