A locknut having a body portion with an outer surface and first and second ends, a thread extending inwardly into the body portion from the first end, a depression in the second end, an inwardly tapered lobed curvilinear surface defining the periphery of the depression for receiving a mating key, and an external thread extending from the second end onto the outer surface for receiving the internally threaded surface of a sleeve mounted on a key for attaching the key to the locknut. The locknut as described above including a rotatable sleeve mounted thereon with a shroud located radially outwardly of the external thread. A locknut as described above wherein the outer surface extending from the first end is frustoconical and terminates at an annular shoulder which is located radially outwardly of the external thread.