An autoantonym (pronounced auto-ANTA-nim) is a word with two opposite meanings. A familiar example is the Hawaiian word aloha, which means both “hello” and “goodbye.”

Autoantonyms (also known as contranyms, contronyms, and Janus words) are not rare. We see, hear, and use them all the time. Too often, miscommunication ensues.

It’s awful when you think you said “purple” but the whole world heard “green.” The great challenge of speaking and writing is to convey your intended meaning and avoid misunderstandings. This is why autoantonyms, with their split personalities, must be recognized and remedied before they do their mischief. Here are a few examples:

Off  It doesn’t necessarily mean “not operating”: First the lights went off, then the alarm went off. What happened after the lights went off? Did the power outage trigger the alarm system or shut it down?

With  This word can mean “side by side” or “in opposition to.” Maxine fought with Charles to gain custody of her daughter. It is unclear whether Charles was helping or hindering Maxine in her efforts to gain custody.

Finished  Accomplished successfully or ruined? Thanks to my investors, this film is finished. Either the investors’ generosity was instrumental in the film’s completion or their interference doomed the project.

Oversight  It is the act of rigorously keeping your eye on something or negligently taking your eye off something. Your oversight proved to be the difference between success and failure could mean “your diligence was crucial to our success” or “your carelessness caused us to fail.”

Trim  After we trimmed our Christmas tree, it was a perfect fit for the living room. Did the family adorn the tree or prune it?

Left  Who’s left? It can mean “Who has departed?” or “Who is still here?”

Some autoantonyms are phrases, even complete sentences. The expression I could care less has befuddled linguists for decades because it usually means “I could not care less.”

The hipster culture devises autoantonyms to confound society’s mainstream. Throughout most of the twentieth century, jive meant both “jazzy, swinging” and “empty, fraudulent.” For over fifty years, bad and wicked have been hip terms for “great.” More recently, sick has come to mean “ridiculously excellent.” A bomb used to be an embarrassing flop, but all that changed when it’s da bomb! became high praise.

The standard definition of uptight is “inhibited, unable to enjoy life.” But it once meant “as good as it gets.” The singer-songwriter Stevie Wonder scored a big hit with his 1966 album Up-Tight. Would Wonder have chosen an album title that meant “inhibited”?

Slim chance—or, to put it another way, fat chance.