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

Meanwhile, as mentioned above, Apple has a killer lineup of returning shows.

From MarketWatch • May 31, 2026

I love watching comedians navigate that impossible little tightrope between structure and chaos: character work, timing, escalation, commitment and, ideally, a killer ending.

From Salon • May 25, 2026

NAIROBI, Kenya—The fast-spreading Ebola virus is overwhelming creaking hospitals and clinics in the Democratic Republic of Congo, dimming hopes for quick containment of what is already the third-largest outbreak of the killer disease in history.

From The Wall Street Journal • May 24, 2026

Maybe Lauer turns from a Dodger killer into a Dodger asset.

From Los Angeles Times • May 19, 2026

But when a killer whale catches and eats a baby seal, I immediately change it back to the Korean variety show.

From "Finding Junie Kim" by Ellen Oh