Lumbago refers to pain in the low back or lumbar region. A simpler term for the same problem would be "backache".
Backache or lumbago can be caused by muscle strain, arthritis, or disc problems. It's not so much an old-fashioned term as it's one that is applied to older adults who tend to have this problem.
The pain is worse with movement and often restricts movement even more. The person with lumbago complains of extreme stiffness, especially when trying to bend forward or backward.