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Definitions

sleeper

[slee-per] / ˈsli pər /
NOUN
dark horse
Synonyms
STRONGEST
also-ran long shot underdog
WEAK
hundred-to-one shot improbability little chance little opportunity outside chance outside shot poor bet poor lookout poor possibility poor prognosis poor prospect slim chance small chance stalking horse unexpected winner unknown unknown quantity unlikelihood unlikely winner upsetter
NOUN
late bloomer
Synonyms
WEAK
naïve person slowpoke up-and-comer
Antonyms
WEAK
child prodigy
NOUN
nightgown
Synonyms
STRONGEST
lingerie pajamas
STRONG
negligee nightdress nightie nightshirt
WEAK
PJs bedgown nightrobe
NOUN
pajamas
Synonyms
STRONG
PJ's jammies jams loungewear nightdress nightie nightshirt nightwear
WEAK
jamas lounging robe sleeping suit
NOUN
sleeping car
Synonyms
WEAK
Pullman car roomette sleeper car wagon-lit
NOUN
spy
Synonyms
STRONGEST
agent detective informer investigator mole operative secret agent secret service undercover agent
STRONG
emissary lookout observer patrol plant scout sleuth snoop spotter watcher
WEAK
double agent espionage agent foreign agent inside agent intelligencer plumber

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

"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

Teo was a good sleeper and she enjoyed the days spent caring for her baby son while on maternity leave.

From BBC • Mar. 13, 2026

AI systems can harbor sleeper agents, trained to behave perfectly under evaluation and activate under specific conditions without detection.

From The Wall Street Journal • Mar. 2, 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

Perhaps, I realize, he wasn’t really such a sound sleeper after all.

From "The One and Only Ivan" by Katherine Applegate

From Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.