What's impugn mean, from Ivanhoe?

To impugn (rhymes with immune) is to attack by argument or criticism; oppose or challenge as false or questionable.

In Ivanhoe, the Grand Master of the Knights Templar describes one of the knights:

[Brother Brian] . . . hath become an active and earnest agitator, a murmurer, and a machinator, and a leader amongst those who impugn our authority . . .