sick ‧ o /ˈsɪkəʊ $ -koʊ/ BrE AmE noun ( plural sickos ) [countable] informal
[ Word Family: noun : ↑ sick , the sick, ↑ sickness , ↑ sicko ; adjective : ↑ sick , ↑ sickening , ↑ sickly ; adverb : ↑ sickeningly , ↑ sickly ; verb : ↑ sicken ]
someone who gets pleasure from things that most people find unpleasant or upsetting:
What kind of sicko would write something like that?