wallet neuropathy
n. Lower back pain caused by sitting on an overstuffed wallet kept in a back pants pocket.

Example Citations:
Physiotherapists have coined the term 'wallet-neuropathy' for the lower back pain caused by men sitting down (such as when driving or in the office) on wallets always carried in their back trouser pocket.

The condition is triggered by the wallet pressing on sciatic nerves in the back and can lead to pain or numbness in the lower leg, ankle or foot.
—Victoria Lambert, "Why marrying a younger woman can save your life (even if it sends HER to an early grave)," Daily Mail, June 30, 2009

The condition is a form of sciatic neuropathy, since it affects the sciatic nerve. But it's becoming so common that it has even been given other names — hip-pocket syndrome or wallet neuropathy.
—Abaidullah, "Wallets and Back Pain," Scribd, June 16, 2008

Earliest Citation:
According to experts, there's an explosion of 'wallet neuropathy' as men damage key nerves by sitting down with their wallets in their back pocket.

Take heart, chaps, there's a simple cure — get your wallet out more often.
—"Barometer," Daily Mail, January 27, 2003

Wallet neuropathy is also called wallet neuritis, wallet sciatica, fat wallet syndrome, and hip-pocket syndrome. However, my favorite is credit carditis (presumably because the wallet is overstuffed with credit cards), which has been around since at least 1965:

"Back-pocket sciatica," caused by carry thick wallets or golf balls in the hip pocket (with a variation called "credit carditis" caused by carry a wallet [stuffed with credit cards])."
—"RQ", American Library Association Adult Services Division, 1965

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