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

Derived Forms

• self-killer
  noun

Etymology

Origin of killer

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

Example Sentences

Yet as more data arrives, Summit’s drug is looking less a Keytruda killer than a contender for a slice of its market.

From The Wall Street Journal • Jun. 6, 2026

“I think this would be a great opportunity to just show that personalized Siri is the killer consumer agent,” John Belton, portfolio manager at Gabelli Funds, tells me.

From Barron's • Jun. 5, 2026

Walmart has been a killer stock for the better part of this decade, but it’s now struggling to keep pace with its biggest rivals and the overall market.

From MarketWatch • Jun. 4, 2026

For decades, scientists believed that MHC class I molecules primarily communicated with CD8+ T cells, often called "killer" T cells, while MHC class II molecules activated CD4+ T cells, commonly known as "helper" T cells.

From Science Daily • Jun. 4, 2026

Anyway, you would be surprised by how many people died in the killer skunk attacks of 1919, including poor Billy Snagglehook, May God Rest His Soul.

From "How to Disappear Completely" by Ali Standish