Etymology: A portmanteau of the words “friend” and “enemy,” frenemy is thought to have been coined in 1953 when American gossip columnist Walter Winchell suggested applying it to the fraught relationship between Russia and the United States.

Meaning: Have you ever spent the day with an acquaintance only to feel oddly deflated on the ride home? Or found yourself blindsided in the workplace by a smiling colleague delivering a scorpion-tailed remark? Scan your body to see whether you’re experiencing the telltale symptoms—racing heart, flushed face, urge to strangle this person—brought on by the presence of a chum whose intentions are distinctly un-chummy. Perhaps the diagnosis will come back affirmative: Here stands a frenemy.

Used to describe a superficially amicable relatio...