English Reference
noun

sleeper

n.
US /ˈsɫipɝ/ UK /slˈiːpɐ/ sleep·er UK spelling sleepered, Sleepers

  1. 01 A2 Elementary

    a person who is sleeping, often described by how well they sleep.

    a person who is in a state of sleep, typically categorized by the quality or depth of their rest.

    Simple

    I am a very heavy sleeper, so the loud thunderstorm didn't wake me up.

    In context

    As a notoriously light sleeper, she found it impossible to rest in the city center due to the constant hum of traffic.

    slumberer
  2. 02 B1 Intermediate

    a train carriage that has beds for passengers to sleep in during a long journey.

    a railroad car equipped with berths or compartments for passengers to sleep in.

    Simple

    We booked a sleeper for the overnight trip from London to Edinburgh.

    In context

    Traveling by sleeper allowed the consultants to arrive in the capital refreshed and ready for their morning presentations.

    sleeping carpullman
  3. 03 B2 Upper Intermediate

    a book, movie, or product that becomes a big success after being ignored for a long time.

    a commercial or artistic work that achieves significant success only after a period of relative obscurity.

    Simple

    The low-budget horror movie was the sleeper hit of the summer.

    In context

    Industry analysts were stunned when the indie title became a sleeper success, eventually outselling the year's most anticipated blockbusters.

    dark horsesurprise hit
  4. 04 B2 Upper Intermediate

    a spy who lives a normal life in a country and waits for a long time before starting their secret work.

    an agent or spy placed in a target country or organization to remain dormant until activated for a specific mission.

    Simple

    The police discovered that the quiet neighbor was actually a sleeper for a foreign agency.

    In context

    Intelligence officials suspected that several sleepers had been embedded within the tech firm years before the data breach occurred.

  5. 05 B2 Upper Intermediate British English

    the heavy pieces of wood or concrete that support the metal tracks of a railway.

    a rectangular support for the rails in railroad tracks, typically made of treated wood, concrete, or steel.

    Simple

    Workers were busy replacing the old wooden sleepers with new concrete ones.

    In context

    The derailment was attributed to the degradation of the wooden sleepers, which had rotted after decades of exposure to the elements.

    tierailroad tie
  6. 06 C1 Advanced Slang

    a car that looks like a normal, slow car but has a very powerful engine inside.

    a vehicle that has been high-performance modified but retains an unassuming, stock exterior to deceive others about its speed.

    Simple

    His old family sedan is a total sleeper; it can beat most sports cars in a race.

    In context

    He took pride in his sleeper, enjoying the look of shock on other drivers' faces when his rusted hatchback left them behind at the light.

    q car
  7. 07 B2 Upper Intermediate American English

    a one-piece suit for a baby or child to wear while sleeping, often covering the feet.

    a one-piece set of pajamas for an infant or young child, usually featuring enclosed feet.

    Simple

    The baby looked adorable in his blue fleece sleeper.

    In context

    She bought a set of cotton sleepers for the newborn, ensuring they had snap closures for easier diaper changes at night.

    onesiefootie pajamas
  8. 08 C1 Advanced SlangInformal Medicine

    a pill that helps you fall asleep.

    a pharmacological agent, such as a sedative or hypnotic, administered to induce sleep.

    Simple

    He had to take a sleeper to deal with his jet lag after the long flight.

    In context

    The doctor warned against the long-term use of sleepers, citing the risk of dependency and altered sleep cycles.

    sleeping pillsedative

She slept in parks away from the centre of the city to try and to stay hidden, and says no rough sleeper would turn down a bed if it comes with the right support.

BBC News· 18 Aug 2026

It also provides new clothes and sleeping bags to rough sleepers, and runs a food bank.

The Guardian· 15 Aug 2026

Etymology 1

From Middle English sleper, equivalent to sleep + -er.

Etymology 2

Compare Norwegian sleip (“a sleeper (a timber); as adjective, slippery, smooth”). See slape.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

This entry
Origin Middle English sleper
Level B2 · Upper Intermediate
Frequency Frequent
Updated