Abstract: A nut free from inadvertent loosening has an annular lug with a plurality of generally upright grooves formed in an outer peripheral surface of the annular lug at angular intervals so as to divide the annular lug into a plurality of wall segments. Each wall segment is interposed between two upright grooves. At least one of the wall segments is depressed centripetally to reduce its radius, with the other segments remaining undepressed. The annular lug is integral and coaxial with a base which is engageable with a driver. An internally threaded bore is formed through the nut around a central axis thereof. The annular lug provides an upper zone of the internally threaded bore. The internal thread is continuous from end to end and is not intersected by the vertical grooves. The inner radius of each depressed wall segment is less than the inner radius of the undepressed wall segments. The outer radius of each depressed wall segment may be greater than or less than the outer radius of the undepressed wall segments.