Source : Webster's Revised Unabridged Dictionary (1913)

Lumbago \Lum*ba"go\, n. [L., fr. lumbus loin. See {Lumbar}.]
   (Med.)
   A rheumatic pain in the loins and the small of the back.

Source : WordNet®

lumbago
     n : backache affecting the lumbar region or lower back; can be
         caused by muscle strain or arthritis or vascular
         insufficiency or a ruptured intervertebral disc [syn: {lumbar
         pain}]