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English definition of “sit up”

sit up

verb uk phrasal verb with sit   /sɪt/ (present participle sitting, past tense and past participle sat) us  

(STRAIGHT)

to sit with a straight back: How many times do I have to tell you children to sit up straight?

(STAY AWAKE)

to stay awake and not go to bed although it is late: [+ -ing verb] The book was so interesting that I sat up all night reading it. I'll be late tonight, so don't sit up for (= wait for) me.

(NOTICE)

informal to show interest or surprise: The news that he was getting married really made her sit up.
(Definition of sit up phrasal verb, verb from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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