To untuck means to pull something out from where it was tucked in. When you untuck your shirt, you pull it out from inside your pants or skirt so it hangs loose. At bedtime, you might untuck the sheets and blankets that were neatly folded under the mattress so you can climb in. After a formal event where everyone had to look sharp, you might immediately untuck your shirt because it feels more comfortable.

The word comes from removing a tuck, which is when fabric or material is folded and secured in place. People untuck things both for comfort and style: some shirts look better tucked in for school pictures or special occasions, but feel better untucked during recess or after the event is over. You can also untuck your legs if they were folded beneath you on a couch, straightening them out into a more relaxed position.