Yours, Etc.: Origins and Uses of 8 Sign-Offs
1610s, from Latin sonorus "resounding," from sonor "sound, noise," from sonare "to sound" (see sonata). Related: Sonorously; sonorousness. Earlier was sonouse (c.1500), from Medieval Latin sonosus; sonourse "having a pleasing voice" (c.1400), from sonor + -y (2).