He has a bend sinister. He was not born in lawful wedlock. In heraldry, a band running from the upper right-hand corner to the lower left-hand corner (as the shield appears before you on paper) is called a bend-sinister, and is popularly, but erroneously, supposed to indicate bastardy.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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