Coccyx
From The Big Bang Theory Wiki
The human "tailbone", e.g. the final vertebrate which are fused at the base of the spine. The plural is coccyges.
In The Einstein Approximation, Raj is angry that Howard and Leonard twisted his idea of going rollerskating together:
Raj: Okay, just to be clear, rollerskating was my idea and I’m very unhappy that you turned it into a double date and I hope you both fall on your asses and break your coccyxs.
Sheldon: The plural of coccyx is coccyges.
Later in the episode, when they return from rollerskating, Leonard rubs his back and says he might have broken his coccyx, but Penny shouldn't tell Raj (0314).
