killer

American  

[kil-er] / ˈkɪl ər /

noun

killers plural

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.

---

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

---

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

Inflected Forms

Nouns

• killers
  plural

Etymology

Origin of killer

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

Example Sentences

"Killer heat warnings" reads the Metro's headline, as Western Europe's "heat dome" covering Italy, Spain and France radiates across the English Channel.

From BBC • Jun. 24, 2026

Appeared in the June 12, 2026, print edition as 'Texas Ranchers Take On Once-Vanquished Cattle Killer'.

From The Wall Street Journal • Jun. 12, 2026

Selected by Amin Ahmad, the author of the forthcoming novel “A Killer in the Family.”

From The Wall Street Journal • Mar. 30, 2026

Mousavi is one of the brains behind fitness coach Joe Wickes' Killer protein bar, designed as a marketing stunt to draw attention to the ultra-processed nature of so-called health products.

From BBC • Mar. 5, 2026

I don't say anything and Freak is looking at me like he's never seen me before and then Iggy says, "Killer Kane. What a tough hombre he was."

From "Freak The Mighty" by Rodman Philbrick