abettor

n. one who aids or encourages someone to do something (especially something illegal)

Abettor (from to abet, O. Fr. abeter, a and beter, to bait, urge dogs upon any one; this word is probably of Scandinavian origin, meaning to cause to bite), is a law term implying one who instigates, encourages or assists another to commit an offence.An abettor differs from an accessory in that he must be present at the commission of the crime; all abettors (with certain exceptions) are principals, and, in the absence of specific statutory provision to the contrary, are punishable to the same extent as the actual perpetrator of the offence. A person may in certain cases be convicted as an abettor in the commission of an offence in which he or she could not be a principal, e.g. a woman or boy under fourteen years of age in aiding rape, or a solvent person in aiding and abetting a bankrupt to commit offences against the bankruptcy laws.
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Noun
1. one who helps or encourages or incites another
(synonym) abetter
(hypernym) accessory, accessary
(derivation) abet

(n.)
One who abets; an instigator of an offense or an offender.
  

ABET - To encourage or set another on to commit a crime. This word is always taken in a bad sense. To abet another to commit a murder, is to command, procure, or counsel him to commit it.

ABETTOR - One who encourages or incites, persuades or sets another on to commit a crime . Such a person is either a principal or, an accessory to the crime. When present, aiding, where a felony is committed, he is guilty as principal in the second degree, when absent he is merely an accessory.
   

This entry contains material from Bouvier's Legal Dictionary, a work published in the 1850's.