Hypochondriac comes ultimately from the Greek word hypokhondria, which literally means “under the cartilage (of the breastbone).” In the late 16th century, when hypochondriac first entered the English language, it referred to the upper abdomen.
The upper abdomen, it turns out, was thought to be the seat of melancholy at a time when the now-outdated medical theory of the four humors (blood, phlegm, yellow bile [choler], and black bile [melancholy]) was accepted as a basis for legitimate health practice. In the 17th century, hypochondriac referred to people who suffered from “depression and melancholy without cause,” though we might suppose from the name of this malady that many depressed patients complained of abdominal pains, which otherwise went undiagnosed. “Vapors,” another archaic disorder connected to the upper abdomen, was used as a euphemism for PMS in a time when such things were not discussed in polite conversation. Because doctors were male at this time, “women’s problems” were largely written off as fits of hysteria (another obsolete medical term of Greek origin, from the word for womb ).
It wasn’t until the 19th century that hypochondriac described someone who suffered “illness without a specific cause.” This sense is still widely used.

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