Use lumbar for anything to do with the lower back, as in lumbar spine or lumbar support. Use lumber for sawn wood, old unwanted furniture, or moving in a slow, heavy way, as in the bear lumbered across the clearing. The words look alike but describe completely different things.
The core difference
Lumbar is a medical term for the lower back; lumber is wood, clutter, or a heavy way of moving.
- lumbar — relating to the lower back, between the ribs and pelvis: She injured her lumbar spine in the fall.
- lumber — wood sawn into boards or planks: The yard sells lumber for decking and fencing.
- lumber (verb) — to move in a slow, heavy, clumsy way: The bear lumbered across the clearing towards the river.
How to tell them apart
Lumbar comes from Latin lumbus, meaning "loin", and stays close to that root, appearing almost only in medical contexts such as lumbar spine, lumbar support or lumbar vertebrae. Lumber has no such tie to the body; it covers sawn wood, cluttered old belongings gathering dust in an attic, and the slow, heavy movement of something large.
The confusion tends to strike in health or ergonomics writing, where someone describing a chair's lower-back support types lumber support instead of lumbar support, since lumber is the far more common everyday word.
