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.