The Collaborative International Dictionary of English v.0.48:

Aforethought \A*fore"thought`\, a.
   Premeditated; prepense; previously in mind; designed; as,
   malice aforethought, which is required to constitute murder.
   --Bouvier.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Aforethought \A*fore"thought`\, n.
   Premeditation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aforethought
    adj 1: planned in advance; "with malice aforethought" [syn:
           aforethought(ip), planned, plotted]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "aforethought":
   advised, calculatedness, calculation, considered, deliberateness,
   deliberation, designed, express intention, expressness,
   forethought, intentionality, preconsideration, preconsidered,
   predeliberated, predeliberation, predetermination, predetermined,
   premeditated, premeditation, prepense, preresolution, preresolved,
   studied, studious, thought-out



Bouvier's Law Dictionary, Revised 6th Ed (1856):

AFORETHOUGHT, crim. law. Premeditated, prepense; the length of time during
which the accused has entertained the thought of committing the offence is
not very material, provided he has in fact entertained such thought; he is
thereby rendered criminal in a greater degree than if he had committed the
offence without. premeditation. Vide Malice; aforethought; Premeditation 2
Chit. Cr. 785; 4 Bl. Com. 199; Fost. 132, 291, 292; Cro. Car. 131; Palm.
545; W. Jones, 198; 4 Dall. R. 146; 1 P. A. Bro. App. xviii.; Addis. R. 148;
1 Ashm. R. 289.