What is Autrefois Attaint?

Legal Definition
Crim. law. Formerly attainted.

2. This is a good plea in bar, where a second trial would be quite superfluous. Co. Litt. 390 b, note 2; 4 Bl. Com. 336. Where, therefore, any advantage either to public justice, or private individuals, would arise from a second prosecution, the plea will not prevent it; as where the criminal is indicted for treason after an attainder of felony, in which case the punishment will be more severe and more extensive. 3 Chit. Cr. Law, 464.
-- Bouviers Law Dictionary
Legal Definition
In criminal law. Formerly attainted. A plea that the defendant has already been attainted for one felony, and therefore cannot be criminally prosecuted for another. 4 Bl. Comm. 336.
-- Black's Law Dictionary
Legal Definition
Previously attainted.
-- Ballentine's Law Dictionary