Gambler's dirk or prostitute dirk as carried in their garter. I'd say third quarter of the 19th century. I've handled an number of naval dress dirks, but this one strikes me more as a Western-type dagger (many of them were Sheffield-made and sent to the American SW. They fit in nicely with Old West collections). Note this old Cowens auction below. The quillons on the guards of these are typically spherical.
https://www.cowanauctions.com/lot/iv...led-dirk-52302