Abstract: Disclosed is a nut having an internally threaded cylindrical inner wall portion joined at one end by a connecting portion to an outer wall portion and separated by an annular volume. The outer wall portion possesses resilient reentrant fingers located opposite the connecting portion and having terminations that project beyond the inner periphery of the inner wall portion. A bolt has external threads that mate with the internal threads of the inner wall portion and also engage and deform outwardly the terminations of the resilient fingers. Discontinuous locking recesses are disposed at spaced apart locations in the grooves of the external bolt threads. During engagement of the nut, the terminations of the resilient fingers spring into locking position within the locking recesses upon circumferential alignment therebetween and prevent reverse rotation of the nut.