This word is a perfect example of how English is a connotative language. Back in Jane Austen days, it was believed that phlegm was a fluid which gave someone a cold personality, probably because an excess of it stuffs up your nose and makes your voice sound distant and coldl that is, someone who was cold, bitter and cruel was said to have a lot of phlegm. As such, an excess of phlegm slowly came to be called a cold, i.e. a disease in which you cough up a lot of phlegm and sneeze it out and stuff.

Phlegm (?), n. [F. phlegme, flegme, L. phlegma, fr. Gr. a flame, inflammation, phlegm, a morbid, clammy humor in the body, fr. to burn. Cf. Phlox, Flagrant, Flame, Bleak, a., and Fluminate.]

1.

One of the four humors of which the ancients supposed the blood to be composed. See Humor.

Arbuthnot.

2. Physiol.

Viscid mucus secreted in abnormal quantity in the respiratory and digestive passages.

3. Old Chem.

A watery distilled liquor, in distinction from a spirituous liquor.

Crabb.

4.

Sluggishness of temperament; dullness; want of interest; indifference; coldness.

They judge with fury, but they write with phlegm. Pope.

© Webster 1913.