an exercise in which a person lies flat on the back, lifts the torso to a sitting position, and then lies flat again without changing the position of the legs: formerly done with the legs straight but now usually done with the knees bent.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
How to use sit-up in a sentence
It's the equivalent of having a nagging teacher or parent telling you to "sit up!"
That 31 percent number made me sit up straight for one reason.
All night she would sit up sewing, pressing rhinestones, going blind.Mel Brooks Is Always Funny and Often Wise in This 1975 Playboy Interview | Alex Belth | February 16, 2014 | THE DAILY BEAST
It was so hard to sit up there and stay composed and keep track of everything—to process what the facts were.New York Journalists Remember the First Moments of 9/11 | Abby Haglage | September 11, 2013 | THE DAILY BEAST
Those last two points should make folks sit up a little straighter.
The hut was barely high enough to let him sit up, and long enough to let him lie down—not to stretch out.The Giant of the North | R.M. Ballantyne
They had been permitted to sit up till after the ice-cream, which naturally marked the limit of human indulgence.The Awakening and Selected Short Stories | Kate Chopin
By that time the fellow I'd kicked had so far recovered as to sit up, and the look he gave us was a scorcher.Raw Gold | Bertrand W. Sinclair
It is not an easy matter to sit up in a gale of wind, with freezing spray, and sometimes green seas, sweeping over one!The Floating Light of the Goodwin Sands | R.M. Ballantyne
At dusk he left off, saying he should get it done at night, his senses would come then, and he should be glad to sit up.The Daisy Chain | Charlotte Yonge
British Dictionary definitions for sit up
to raise (oneself or another) from a recumbent to an upright or alert sitting posture
(intr) to remain out of bed and awake, esp until a late hour
(intr) informal to become suddenly interested or alert: devaluation of the dollar made the money market sit up
a physical exercise in which the body is brought into a sitting position from one lying on the back: Also: trunk curl
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Other Idioms and Phrases with sit-up
Rise to a sitting position from lying down, as in The sick child sat up and asked for a drink of water. [Early 1200s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.