Definition of sit up (or sit someone up) in English:

sit up (or sit someone up)

phrasal verb

  • 1Move (or cause someone to move) from a lying or slouching to a sitting position.

    ‘Amy sat up and rubbed her eyes’
    ‘I'll sit you up on the pillows’
    • ‘Sit on the cushion with it more to your rear than to your front and bring the body to the best possible position, sitting up quite straight.’
    • ‘It then occurred to me that maybe Josh was uncomfortable with the position we were in, so I moved to sit up.’
    • ‘She walks into the room, and everybody sits up a little straighter and leans forward a little.’
    • ‘With a sigh, the stranger walked over toward Kiro and sat her up straight.’
    • ‘Somehow she managed to sit him up, propping his back against the rough bark of the oak tree.’
    • ‘After another long moment of silence he sat up straighter and moved to stand up.’
    • ‘Cole nodded from his position below her and moved to sit up.’
    • ‘Then he moves, sitting up on the hammock, and speaking in a commanding and surprisingly powerful voice.’
    • ‘The patient is positioned sitting up and leaning forwards over a bed table.’
    • ‘When she realized she was home she sat up straighter and rubbed the sleep out of her eyes.’
    • ‘The men sleeping in the room sat up and moved to make space by the fire for the new arrivals.’
  • 2Refrain from going to bed until a later time than usual.

    ‘we sat up late to watch a horror film’
    • ‘I sat up late last night working on this project for my niece.’
    • ‘He sat up late thinking about it and deep in the middle of the night he wrote to me about his feelings.’