A term in chivalry, meaning to strike the helmet and shield of an antagonist so firstly 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 champion's having
attained his mark, or in other words, struck his lance straight and
fair against the helmet or breast of his adversary.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894