The primary meaning of pest
is an animal
which has characteristics which people regard as being injurious or harmful. It can also refer to plants (weeds). Pesticides are chemicals designed to kill, retard growth, or prevent reproduction of pests.
By extension, pest can refer also to a contagious deadly disease, a pestilence such as the Bubonic plague.
By further analogy, pest in the slang sense can refer to an annoying person, especially one difficult to get rid of.
Pest, with a different pronunciation, is also the name of a county and former city in Hungary.
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