E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A term in chivalry, meaning to strike the helmet and shield of an antagonist so firmly with the lance, held in a direct line, as either to break the lance or overthrow the person struck. Hence to attaint of treason, etc.
Attaint was a term of tilting, used to express the champions having attained his mark, or, in other words, struck his lance straight and fair against the helmet or breast of his adversary.Sir Walter Scott: The Monastery (note).