Sentence:  be assured, O man of sin—pilferer of small wares and petty larcener—that there is an eye within keenly glancing from some loophole contrived between accordions and tin breastplates that watches your every movement, and is "<WSD>fly</WSD>,"— to use a term peculiarly comprehensible to dishonest minds—to the slightest gesture of illegal conveyancing. 
Meaning of the word marked with <WSD> tags:  Quick-witted, alert, mentally sharp.