Peruse: We usually use this word to mean the same thing as “scan” or “skim,” but it actually means the opposite. To peruse is to study thoroughly and carefully. It usually refers to books or papers, but you can peruse a work of art, too.
Bemused & Nonplussed: This pair of words is almost never used correctly. According to Ben Zimmer, executive editor of Vocabulary.com and VisualThesaurus.com, “Here we have two words that have traditionally meant something like ‘bewildered’ or ‘perplexed,’ but they’ve each veered off in different semantic directions — one towards resolute calmness (nonplussed) and the other towards mild amusement (bemused).”
Disinterested: This word and uninterested are often used as synonyms, but they’re not quite the same. To be uninterested is to have no enthusiasm for something, while to be disinterested is to have no bias. For example, if you were on trial, you’d want an unbiased, disinterested judge, not one who was bored by your case.
Flammable & Inflammable: Unlike the other word pairs on this list, these actually mean the same thing even though they’re often mistaken for antonyms. Since the early 20th century, flammable has been the preferred spelling. According toGrammarist, “[b]ecause this confusion can have dangerous real-world consequences, the shift from inflammable to flammable is welcome.”
Nauseous & Nauseated: One means to cause nausea and the other means to experience it, but do you know which is which? If someone is feeling sick, they’re nauseated — but wait to correct their grammar until you’ve reached a safe distance.
Moot: You’ve probably heard the phrase “It’s a moot point,” meaning that the issue isn’t worth discussing. The primary definition actually describes a topic or question that’s open for discussion, but which may be theoretical or academic. Moot is derived from an Old English word for a political meeting; die-hard Lord of the Rings fans may recognize it from the “Entmoot,” the gathering of the ancient, tree-like creatures to debate whether to go to war.
Decimate: The word decimate is usually used to mean complete and utter destruction. But did you know that it originally meant to reduce by ten percent? In Roman times, when a cohort of soldiers was found guilty of misconduct, it wasn’t practical to execute all 480 of them. Instead, they were broken into groups of ten and made to draw lots. Drawing the short straw meant a painful death at the hands of your comrades.
Ultimate: This word gets trotted out as a synonym for “extreme” or “best of the best,” but it originally meant the last or final occurrence of something. (Incidentally, penultimate is the next-to-last thing, as in the penultimate episode of a TV show before the series finale.)
Enormity: Most people think this word describes something huge or momentous. It makes sense–enormity sounds a lot like enormous, after all — but its primary definition is not bigness but badness. Merriam-Webster defines it as “a shocking, evil, or immoral act” or a “great evil or wickedness.” However, the persistent misuse of the word has caused the definition to expand and shift over time.
Sticklers may find these shifts in meaning to be frustrating, but that’s what happens to living languages. Grammar changes over time, beginning with the most casual speech and eventually causing shifts in even formal academic language. Since different rules apply for different audiences, proofreading can be a difficult task. When in doubt, double-check the dictionary — or the thesaurus!