plural rats
1
: a small animal that has a pointed nose and a long, thin tail
a dirty old building infested by rats and mice
a lab/laboratory rat [=a rat that is used by scientists for experiments]
He came in from the rain looking like a drowned rat. [=looking very wet and miserable]
2
informal
a
: a person who is not loyal or who cannot be trusted
I can't believe that rat turned us in to the police!
b
: a person who is bad or cruel
No one understands why she's with a rat like him.
3
US, informal
: a person who spends a lot of time in a specified place
a mall rat
Every night he goes to work out with the other gym rats.
rats; ratted; ratting
informal
: to tell someone in authority (such as the police) about something wrong that someone has done : to betray someone
The teacher knows what we did, which means that somebody ratted.
—often + onI am trying to find out who ratted on me.
(US)
—+ outSomeone ratted him out to the police.
They accused him of ratting out his own brother.
Cite this Entry
“Rat.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/rat. Accessed 23 Jun. 2026.
Copy Citation
