intransitive verb
Etymology: Middle English sitten up, from sitten to sit + up — more at sit , up
1. : to rise from a lying to a sitting position : sit with the body erect
able to sit up and take nourishment
2. : to show interest or alertness or surprise
we sit up with pleasure when he suddenly lashes out at some literary stuffed shirt — Alfred Kazin
3. : to stay up after the usual time for going to bed
sit up with a sick child
a late movie not worth sitting up for
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- sit up and take notice