Bar,

in heraldry. An honourable or dinary, consisting of two parallel lines drawn across the shield and containing a fifth part of the field.

“A barre … is drawne overthwart the escochon … it containeth the fifth part of the Field.”—Gwillim: Heraldry.

A Bar sinister in an heraldic shield means one drawn the reverse way; that is, not from left to right, but from right to left. Popularly but erroneously supposed to indicate bastardy.

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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.