Sentence:  By not knowing it: the fellow's manners are so plausible and even insinuating, his appearance so gentlemanly, his voice so winning, that I suffered him to cheat me at Newmarket in the sale of a spavined horse, a regular screw, on the strength of which he claimed my acquaintance ; but as soon as I twigged that he was a <WSD>flashman</WSD> — one of the swell mob, I mizzled — gave him the cut direct. 
Meaning of the word marked with <WSD> tags:  Someone who distracts the target of a thief; a confidence trickster.