The Collaborative International Dictionary of English v.0.48:

Lumbago \Lum*ba"go\, n. [L., fr. lumbus loin. See Lumbar.]
   (Med.)
   A rheumatic pain in the loins and the small of the back.
   [1913 Webster] Lumbar

WordNet (r) 3.0 (2006):

lumbago
    n 1: 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:
         lumbago, lumbar pain]

Moby Thesaurus II by Grady Ward, 1.0:

200 Moby Thesaurus words for "lumbago":
   abscess, adenoiditis, adrenitis, ague, anemia, ankylosis, anoxia,
   apnea, appendicitis, arteritis, arthritis deformans,
   arthritis fungosa, arthritis pauperum, asphyxiation, asthma,
   ataxia, atrophic arthritis, atrophic inflammation, atrophy,
   backache, bleeding, blennorhea, blennorrhagic arthritis,
   brain fever, bronchitis, bunion, bursitis, cachexia, cachexy,
   capillaritis, carditis, catarrh, catarrhal inflammation,
   cerebellitis, cerebral meningitis, cerebritis,
   cerebrospinal meningitis, chill, chills,
   chronic infectious arthritis, chronic inflammation,
   cirrhotic inflammation, climactic arthritis, clitoritis, colic,
   colitis, collagen disease, conjunctivitis, constipation,
   convulsion, coughing, cyanosis, cystitis, degenerative arthritis,
   diarrhea, diffuse inflammation, dizziness, dropsy, dysentery,
   dyspepsia, dyspnea, edema, emaciation, encephalitis, endocarditis,
   enteritis, equine encephalomyelitis, exudative inflammation,
   fainting, fatigue, fever, fibrillation, fibroid inflammation, flux,
   focal inflammation, gastritis, gingivitis, glossitis,
   gonococcal arthritis, gonorrheal arthritis, gonorrheal rheumatism,
   gout, gouty arthritis, growth, hemophilic arthritis, hemorrhage,
   hepatitis, high blood pressure, hydrops, hyperplastic inflammation,
   hypertension, hypertrophic arthritis, hypertrophic inflammation,
   hypotension, icterus, indigestion, infectional arthritis,
   infectious hepatitis, inflammation, insomnia,
   irritable bowel syndrome, itching, jaundice, labored breathing,
   laryngitis, low blood pressure, lumbar rheumatism, marasmus,
   mastoiditis, meningitis, menopausal arthritis,
   metastatic inflammation, metritis, milk leg, mucous colitis,
   mumps meningitis, myelitis, nasal discharge, nausea, necrosis,
   necrotic inflammation, nephritis, neuritis,
   obliterative inflammation, ophthalitis, ophthalmia, orchitis,
   osseous rheumatism, osteitis, osteoarthritis, osteomyelitis,
   otitis, paradental pyorrhea, paralysis, penitis, pericarditis,
   periodontitis, peritonitis, pharyngitis, phlebitis, podagra,
   proliferative arthritis, prostatitis, pruritus, pyonephritis,
   pyorrhea, pyorrhea alveolaris, rash, reactive inflammation, rheum,
   rheumatism, rheumatiz, rheumatoid arthritis, rhinitis,
   sclerosing inflammation, sclerosis, seizure,
   seroplastic inflammation, serous inflammation, serum hepatitis,
   shock, simple inflammation, sinusitis, skin eruption, sneezing,
   sore, spasm, spastic colon, specific inflammation,
   subacute rheumatism, suppurative arthritis,
   suppurative inflammation, syphilitic arthritis, tabes, tachycardia,
   tennis elbow, testitis, thrombophlebitis, tonsilitis, torticollis,
   toxic inflammation, traumatic inflammation, tuberculous arthritis,
   tuberculous rheumatism, tumor, ulcerative colitis, upset stomach,
   uratic arthritis, ureteritis, urethral arthritis, urethritis,
   uteritis, vaginitis, vertebral arthritis, vertigo,
   visceral rheumatism, vomiting, vulvitis, wasting, wryneck