I had never heard of this before, but was intrigued to learn about contranyms, words that are their own antonyms, their own opposites. Apparently, contronyms are special cases of homographs (two words with the same spelling). Here’s a couple examples:

  • bolt – It can mean “to secure,” and it can also mean “to run away.”
  • fast – May mean “quick” or “unmoving.”
  • help – Could be “assist,” or “prevent” (e.g., “I can’t help it if…”).
  • screen – May mean “show” (like a movie screening), or “hide.”

What a strange fluke of language! There’s more examples at this fun with words page.