From Longman Dictionary of Contemporary Englishratrat1 /ræt/ ●●○ noun [countable] 1 HBAan animal that looks like a large mouse with a long tail2 spokenBAD PERSON someone who has been disloyal to you or deceived you But you promised to help us, you rat!3 → look like a drowned rat4 → (like) rats leaving the sinking ship → rat race, rats, → smell a rat at smell2(7)
Examples from the Corpus
rat• Old steamboats with their scrollwork more scrolled and withered by weathers sat in the mud inhabited by rats.• He ran on laughing, used to having rats thrown at him.• This is a good way to get back at that rat Yossarian.• The electrodes gave the rats access to their own stores of bliss-producing neurotransmitters.• But the immature brain cells were the most effective treatment in the rats, says Sandberg.• The rat weights did not differ between the groups.• The rats eventually managed to press the lever very quickly after being placed in the box, in order to receive their reward.• The rats must have learned the maze earlier and were demonstrating latent learning.ratrat2 verb (ratted, ratting) [intransitive] informal 1 UNFAITHFULif someone rats on you, they tell someone in authority about something wrong that you have done SYN grassrat on They’ll kill you if they find out you’ve ratted on them!2 British English to not do what you had promised to do SYN go back on, renege onrat on He accused the government of ratting on its promises to the disabled. → rat somebody ↔ out→ See Verb tableExamples from the Corpus
rat• I have to rat all he has reet.• After that you ratted or rotted.• Looks like Rico got in on it, then decided like he was gon na rat to Spider.rat on• I wouldn't rat on Albert.• Can you believe he ratted on his own brother?Origin rat1 Old English ræt 