1
a
: any of numerous rodents (Rattus and related genera) differing from the related mice especially by considerably larger size
b
: any of various similar rodents
2
: a contemptible person: such as
a
: one who betrays or deserts friends or associates
b
c
3
: a pad over which a woman's hair is arranged
4
: a person who spends much time in a specified place
a mall rat
ratted; ratting
1
: to betray, desert, or inform on one's associates
—usually used with on
2
: to catch or hunt rats
3
: to work as a scab
1
: to give (hair) the effect of greater quantity (as by use of a rat)
2
: to inform on : turn in
—usually used with out
ratted out his accomplice
Synonyms
Examples of rat in a Sentence
Noun
a dirty old building infested by rats and mice
I can't believe that rat turned us in to the police!
No one understands why she's with a rat like him.
Every night he goes to work out with the other gym rats.
Verb
The teacher knows what we did, which means that somebody ratted.
Recent Examples on the Web
Glenn Garner, Deadline, 27 Jan. 2025
Research has shown that Red No. 3 can cause cancer in laboratory rats when exposed to high levels.
—Brittney Melton, NPR, 21 Jan. 2025
Regular aerobic exercise was linked to a reduction in markers in the brains of in rats that associated with the onset of Alzheimer's, including amyloid plaques, according to a new paper in the journal Brain Research.
—Michael Gfoeller and David H. Rundell, Newsweek, 16 Jan. 2025
The decision, lauded by consumer advocacy groups, comes a full 25 years after scientists at the agency determined that rats fed large amounts of the artificial color additive were much more likely to develop malignant thyroid tumors than rats who weren’t given the food coloring.
—Ryan Fonseca, Los Angeles Times, 16 Jan. 2025
See all Example Sentences for rat 
Examples are automatically compiled from online sources to
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Opinions expressed in the examples do not represent those of Merriam-Webster or its editors.
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Noun
Shanks brilliantly paces the story out with several pieces of foreshadowing, including a rat king as symbolism for toxic codependency, mirroring a gruesome story Tim tells Millie about his father’s death and his mother’s mental health.
—Glenn Garner, Deadline, 27 Jan. 2025
Research has shown that Red No. 3 can cause cancer in laboratory rats when exposed to high levels.
—Brittney Melton, NPR, 21 Jan. 2025
Regular aerobic exercise was linked to a reduction in markers in the brains of in rats that associated with the onset of Alzheimer's, including amyloid plaques, according to a new paper in the journal Brain Research.
—Michael Gfoeller and David H. Rundell, Newsweek, 16 Jan. 2025
The decision, lauded by consumer advocacy groups, comes a full 25 years after scientists at the agency determined that rats fed large amounts of the artificial color additive were much more likely to develop malignant thyroid tumors than rats who weren’t given the food coloring.
—Ryan Fonseca, Los Angeles Times, 16 Jan. 2025
See all Example Sentences for rat
Dictionary Entries Near rat
Cite this Entry
“Rat.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/rat. Accessed 31 Jan. 2025.
Copy CitationKids Definition
rat
1 of 2 noun1
: any of various rodents that have brown, black, white, or grayish fur and a long usually nearly hairless tail and that look like but are larger than the related mice
2
: a person who betrays friends
3
: a person who spends much time in a specified place
a mall rat
rat
2 of 2 verb
ratted; ratting
1
: to betray, desert, or inform on one's friends
didn't rat on us
ratted them out
2
: to catch or hunt rats
Medical Definition
rat
noun
: any of the numerous rodents (family Muridae) of Rattus and related genera that differ from the murid mice by their usually considerably larger size and by features of the teeth and other structures and that include forms (as the brown rat, the black rat, and the roof rat) which live in and about human habitations and in ships, have become naturalized by commerce in most parts of the world, and are destructive pests consuming or destroying vast quantities of food and other goods and acting as vectors of various diseases (as bubonic plague)
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