If you mean a sit-up as in the exercise, the plural is sit-ups.
Get up/Wake up/Stand up/Sit up
sit down come over to
sit
If you mean the stretch you: sit down on the ground and pull your feet up to your crotch and stretch:)
"Siéntense" and "Siéntese" mean "Sit" (the command).
they sit on the bench
Don't sit up.
Monica does 5 turtles worth of sit-up
"Assieds-toi" in French translates to "sit down" in English. It is a command form instructing someone to take a seat.
you mean a$$hole.... what do you sit on when at the computer, what do you sit on when your on the john?
* Parachute rigger. You pretty much sit in "A" school forever and then you sew up chutes.