Sentence:  As to being taken up, himself, for a rioter, and punished with the rest, Mr Dennis dismissed that possibility from his thoughts as an idle <WSD>chimera</WSD>; arguing that the line of conduct he had adopted at Newgate, and the service he had rendered that day, would be more than a set-off against any evidence which might identify him as a member of the crowd: ... 
Meaning of the word marked with <WSD> tags:  A foolish, incongruous, or vain thought or product of the imagination.