killer

American  

[kil-er] / ˈkɪl ər /

noun

1. a person or thing that kills.

2. killer whale.

3. a device used by a post office for printing cancellations on postage stamps.

4. a mark of cancellation made on a postage stamp.

5. Slang. something or someone having a formidable impact, devastating effect, etc..

   The math test was a real killer.

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adjective

Slang.

1. severe; powerful.

   a killer cold.

2. very difficult or demanding.

   a killer chess tournament.

3. highly effective; superior.

   a killer recipe for fried chicken.

killer British  

/ ˈkɪlə /

noun

1. 1. a person or animal that kills, esp habitually

   2. ( as modifier )

      a killer shark

2. something, esp a task or activity, that is particularly taxing or exhausting

3. an animal selected to be slaughtered for food

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Usage

What is a basic definition of killer? A killer is a person or thing that kills, meaning they end lives. Killer is also used in slang to mean something that is devastating or exhausting. Additionally, killer is used in slang to describe something as severe or powerful. Killer has several other meanings as a noun and an adjective. A person or thing that ends someone’s life is a killer. In terms of a person, killer is often used interchangeably with murderer. However, killer can refer to someone who killed another person by accident, while murderer is not used in that sense.

• Real-life examples: John Wilkes Booth was Abraham Lincoln’s killer. Jack the Ripper was an infamous killer. Bacteria and viruses often act as natural killers. People use weed killers on their lawns.
• Used in a sentence: My cat is a very effective killer of birds.

In slang, killer is used to describe something as causing stress, exhaustion, pain, or misery.

• Real-life examples: You might call a particularly hard test a killer test. A killer workout would push your body to the limits of its abilities.
• Used in a sentence: That history assignment turned out to be a killer and took me forever to finish.

In slang, killer is also used to describe something as severe, intense, or powerful.

• Real-world examples: You might call a baseball game that goes into extra innings with a 2–2 score, a killer game. If you come down with a severe cold that keeps you in bed for several days, you could call it a killer cold.
• Used in a sentence: After living abroad for five years, Michelle came down with a killer case of homesickness.

Other Word Forms

• self-killer noun

Etymology

Origin of killer

First recorded in 1525–35; kill ¹ + -er ¹

Example Sentences

In any case, popes don’t cheer wars, which are always a calamity—a result of failure and a killer of the innocent.

From The Wall Street Journal • Apr. 16, 2026

In response, Lancashire Constabulary's Chief Constable Sacha Hatchett said the force accepted there was an "opportunity" to arrest the killer that day and added: "I am extremely sorry for this."

From BBC • Apr. 13, 2026

Then the stage lit up to show a carved-out cave in a cliffside, housing an absolutely killer all-femme backing band in the grotto and a full company of dancers in every hue of the Americas.

From Los Angeles Times • Apr. 13, 2026

“But the social aspect has to come with some innate awareness of what you’re doing. This is a big killer on pace of play.”

From The Wall Street Journal • Apr. 9, 2026

The fresh-faced blond woman, with cheeks as delicate as a china doll’s, was in cahoots with a killer?

From "The Detective's Assistant" by Kate Hannigan