It is definitely a pocket watch winding key. On yours, the thin shaft which fit into the watch's keyhole is almost entirely broken off. The ring was for attaching the key onto the pocketwatch's chain.
I suspect the next question will be "How old is it?" Key-winding pocketwatches date from the late-1500s onward, and are still being manufactured today. However, the key-winding type fell out of public favor in the 1870s... due to the invention (in 1842) and commercial production (in the 1850s) of stem-winding pocketwatches, which eliminated the need for not losing the winding-key. The "style" of your brass winding-key (ornate scenes on both sides of the octagonal part of its body) indicates it is from sometime in the 1700s into the 1870s. I think yours is most likely late-1700s though early-1800s.