The full
origin of the word parasite is somewhat amusing. It began as the respectable
Greek word
parasitos, meaning quite mundanely a 'fellow-
diner' or 'guest' to the house. This meaning it kept until the
Old Comedy of
Greece became fashionable in the 400s
BCE, when plays involving
rude,
sniveling, or otherwise unpleasant dinner guests were popular. The word concurrently began to gain a sense of '
freeloader', someone who sponged off his host's
hospitality with flattering and
humor while hiding ulteriour motives. The parasitos was a sort of
charming vagabond.
Images of the parasitos continued to appear through Middle Comedy and he became a stock, stereotypical character by the time of the New Comedy. Many times real-life members of government or society would be cast into the role of the parasitos; a technique which served the common goal of many Greek plays to satire. The Latins borrowed the word, like many things Greek, and transformed it into parasitus. Their plays also featured this stock character, though not to the same degree. From Latin to French, from French to English, and there you have it. Parasite!
Howatson, M. C. The Oxford Companion to Classical Literature, 2nd ed. Oxford: Oxford University Press, 1989.