From Longman Dictionary of Contemporary Englishundressun‧dress1 /ʌnˈdres/ ●●○ verb [intransitive, transitive]TAKE OFF CLOTHESto take your clothes off, or take someone else’s clothes offMatt undressed and got into bed.Joe still needs an adult to undress him.RegisterIn everyday English, people usually say get undressed rather than undress:You can get undressed in here.THESAURUSundress/get undressed to take off all your clothes, especially before going to bed. Get undressed is the usualphrase to use in everyday EnglishShe got undressed and went to bed.Em felt uncomfortable undressing in front of other people.take something off/remove to remove a piece of clothing. Remove is more formal than take off'It’s warm in here, ' said Michael, taking off his jacket.Take your shoes off before you come in.Police officers told him to remove his clothes.strip (off) to take off all or some of your clothes – used especially when you do this quickly, when someone has ordered you to do it, or in a way that is deliberately sexually excitingBen stripped off and got into the shower.She stripped off her clothes and lay on the bed.The women in the club screamed and clapped as the male dancers began to strip.get changed to take off your clothes and put on different clothesThe boys ran up the stairs to get changed.I’ll be ready in a second, I just have to get changed.undo to open a piece of clothing that is fastenedHe started to undo his shirt.Amy was struggling to undo her belt.→ See Verb table
undressundress2 noun [uncountable]formalDCTAKE OFF CLOTHES when you are wearing few or no clothesCindy was wandering about her room in a state of undress.
Examples from the Corpus
undress• In the meantime, women paraded back and forth in variousstages of undress.• Men here, in various stages of undress, were pulling on outer clothing and armour.• In Por Tanssie, no one would have marched into a roomuninvited in case the personinside was in a state of undress.• Inevitably they would be in an arrestable state of undress.in a state of undress• Next morning Clarissa was wandering about her bedroomin a state of undress, feeling miserable.• A group of revolutionaryofficers had been drinking and were in the room with her, some in a state of undress.• In Por Tanssie, no one would have marched into a room uninvited in case the person inside was in a state of undress.