"listen up" is a phrasal verb, the "up" alone has no specific meaning, chatGPT is correct on this one 😊 I would say that when speaking to a group of people for example, it's more common to hear someone say "listen up", than just "listen", when giving the command to listen, as "listen" alone can seem a little rude/too direct.

The "up" in "listen up" is an intensifier used to add emphasis and urgency, making the command stronger. It's similar to other phrases like "hurry up" or "clean up," where "up" adds a sense of immediacy.

"Listen up" sounds rude to me. I would use it only if I felt a need to force someone to listen. If I want to be polite, I say "please listen". "Listen" is only rude if you make it so. It all depends on your tone of voice. If you say "listen Mary, I have something to tell you", whether it is polite or rude depends how you say it and the reason why you are saying it.

Yes, UP used this way is a little confusing maybe. As someone said, this is mostly used by a person in aurhority (boss, teacher, coach, military officer) when speaking to a group of subordinates. Still, UP is odd. Here are some other examples. Mount up: get on your horse, into your vehicle, etc. Gear up: (often military) get your equipment and get ready to move. Saddle up: like mount up, but specifically for horses. Lawyer up: Realize that your legal situation is serious and get an attorney; stop answering questions freely without legal advice.

"Listen up" is a command or an imperative. This term is used to tell people it is important to pay attention to what is about to be said RIGHT NOW. It is an urgent and commanding statement usually made by someone in a position of authority over others, such as a boss or elder. You may hear "Listen up!" when someone is trying to get the attention of a crowd during a serious or emergency situation because they have important information to share about that situation. "Listen up" is often said in a quick and sharp or brusque or abrupt or snappy tone. And sometimes loud. (Yes, it could be said in a soft and relaxed way but that is unusual and such a tone could mean something less serious is about to be said instead). This term can be considered rude when said by someone who is not in a position of authority to someone who is. The word "up" is indeed an intensifier. It is meant to tell people to stop what they are doing and pay attention to something else NOW. Compared to "Listen up," simply saying "Listen" is less urgent and may even be ignored. Therefore "listen up" should be used only when a situation calls for it. "Listen up" usually means "Pay attention NOW." Similar but different terms that add the word "up" to urgently encourage an immediate response or action are "Heads up!" which usually means something is being thrown at your head so protect your head (weird I know 😂) and "Look up!" which could mean look up OR just look up from what you're currently doing at something else.