A metal instrument which is fitted to a man's fist, and may be readily used in self-defence by striking a blow. Sometimes these instruments are armed with spikes. It was an American invention, and was used in England in defence against the infamous attacks of Spring-heel Jack. We have the phrase “To dust your jacket for you,” meaning to “beat you,” as men dust carpets by beating them.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894