sleeper

American  

[slee-per] / ˈsli pər /

noun

1. a person or thing that sleeps.

2. a heavy horizontal timber for distributing loads.

3. Building Trades.

   1. any long wooden, metal, or stone piece lying horizontally as a sill or footing.

   2. any of a number of wooden pieces, laid upon the ground or upon masonry or concrete, to which floorboards are nailed.

4. a sleeping car.

5. Informal. something or someone that becomes unexpectedly successful or important after a period of being unnoticed, ignored, or considered unpromising or a failure.

   The play was the sleeper of the season.

6. merchandise that is not quickly sold because its value is not immediately recognized.

7. Often sleepers. one-piece or two-piece pajamas with feet, especially for children.

8. bunting.

9. a sofa, chair, or other piece of furniture that is designed to open up or unfold into a bed; convertible.

10. Also called sleep, sand.  a globule that forms at the inner corner of the eye, especially during sleep, from the accumulated secretion of the glands of the eyelid.

11. any of several gobioid fishes of the family Eleotridae, of tropical seas, most species of which have the habit of resting quietly on the bottom.

12. Slang. a spy; mole.

13. Slang. a juvenile delinquent sentenced to serve more than nine months.

14. Bowling. a pin that is hidden from view by another pin.

15. Chiefly British. a timber or beam laid in a railroad track, serving as a foundation or support for the rails; tie.

---

sleeper British  

/ ˈsliːpə /

noun

1. a person, animal, or thing that sleeps

2. a railway sleeping car or compartment

3. US and Canadian equivalent: tie.  one of the blocks supporting the rails on a railway track

4. a heavy timber beam, esp one that is laid horizontally on the ground

5. a small plain gold circle worn in a pierced ear lobe to prevent the hole from closing up

6. a wrestling hold in which a wrestler presses the sides of his opponent's neck, causing him to pass out

7. an unbranded calf

8. Also called: sleeper goby.  any gobioid fish of the family Eleotridae, of brackish or fresh tropical waters, resembling the gobies but lacking a ventral sucker

9. informal a person or thing that achieves unexpected success after an initial period of obscurity

10. a spy planted in advance for future use, but not currently active

---

Etymology

Origin of sleeper

Middle English word dating back to 1175–1225; see origin at sleep, -er ¹

Explanation

A sleeper is something that succeeds when no one thought it would. That low-tech movie about shark attacks that you shot on vacation and edited in your basement? It could be a sleeper if you market it in the right way. You probably know that sleeper can be used in an obvious sense to refer to someone who is asleep. But you also may hear the word used to describe something that becomes an unexpected success, like "a sleeper of a film" featuring unknown actors that ends up becoming the big summer hit. This sense of the word may have originated in the world of gambling in the nineteenth century. When a card player unexpectedly drew a winning card, the card was called "a sleeper."

Example Sentences

"This suggests that dreaming may reshape how brain activity is interpreted by the sleeper: the more immersive the dream, the deeper the sleep feels."

From Science Daily • Mar. 26, 2026

I am very lucky in that I’m a very good sleeper and the few times in my life when I do experience insomnia, it’s infuriating to me because I am spoiled, basically.

From Los Angeles Times • Feb. 27, 2026

“This was a sleeper issue that people did not pay much attention to before the pandemic. Then things got extremely intense,” Saver said.

From Salon • Jan. 9, 2026

He previously played a KGB sleeper agent in The Americans, for which he won an Emmy Award for his acting in 2018.

From BBC • Jan. 2, 2026

She approached the cacophonous sleeper, now on his side.

From "The Inquisitor's Tale" by Adam Gidwitz

Related Words

• agent
• detective
• informer
• investigator
• mole
• operative
• secret agent
• secret service
• undercover agent