This scene below is from a tv series Modern Family.
Cam caught Jay(father of Mitchell) at a mall being harsh on his stepson, Manny, who was struggling to move an inch at a not-so-tall indoor climbing wall. Cam steps in, telling Jay to stop asking Manny to toughen up, and instead he decides to climb it himself to show and prove Manny that it's actually not so difficult. While Cam's climbing, Jay gets distracted and slipped the harness attached to Cam's climbing chain by accident and Cam falls down to the ground. Later at home, Cam's lying down on the sofa with Jay beside him:
Jay: How you doin'?
Cam: Oh, it's hard to tell. You know, anytime you have a catastrophic injury to your lumbar area ㅡ
Jay: (interrupting Cam, expressionless but sounds a little irritated) I could use a little piece of lumbar myself right now.
I guess Jay's response should be funny to a listener's ears but I don't understand the joke. Dictionary definition of the vocab suggests "relating to the lower part of the back", but then what does it mean by Jay saying "I could use a piece of lumbar region"? (I don't know if the characters' personalities are relevant to understand the context here, but just so you know, Jay is a tough, "manly" guy and Cam is a sensitive, bubbly, outgoing gay.)
Besides, lumbar seems like an adjective. Is "a piece of lumbar" technically, grammatically incorrect?
Thanks in advance!