"having two poles;" see bi- "two" + polar. It is attested from 1810 in the figurative sense of "of double aspect;" by 1859 with reference to anatomy ("having two processes from opposite poles," of nerve cells). Psychiatric use in reference to what had been called manic-depressive psychosis is said to have begun 1957 with German psychiatrist Karl Leonhard. The term became popular early 1990s. Bipolar disorder was in DSM III (1980).
"those who are sick, persons suffering from illness," Old English seoce, from the source of sick (adj.). Colloquial sense of "vomited matter" is by 1959.
"state of being sick or suffering from a disease," Middle English siknesse, from Old English seocnes "sickness, disease; a particular malady;" see sick (adj.) and -ness. It formerly was synonymous with illness; in late 19c. sickness began to be restricted to nausea and other disturbances of the stomach, leaving illness as "a rather more elegant and less definite term" [Century Dictionary].
1640s, "scrutiny into one's own state, conduct, or motives," from self- + examination. By 1955 as "examination of one's own body for signs of illness."
"emotional thrill," 1777 (Walpole), from French frisson "fever, illness; shiver, thrill" (12c.), from Latin frigere "to be cold" (see frigid). Scant record of the word in English between Walpole's use and 1888.
