These are not opposites. No. Nope. Nuh-uh.
But they have very subtle differences depending on who is speaking. A parent can ask their child to “open up” and brush their teeth. But a dentist would say “open wide”.
You can knock on a door and shout “open up” if you’re angry or in a hurry. If you own a shop you can “open up the shop” at 9am, or 10, or 12, your decision.
When you have finished brushing your toddler’s teeth, what do you say? “Close your mouth”. Don’t say “shut up”. When the dentist has finished working on your mouth, they will not say “shut up”.
What does shut up mean? It means be quite, stop talking, stop making a noise. It’s very direct, and maybe a bit rude.
But if you talk about a shop you can say “shut up” when it’s closed for the night, or a long time.
This shop is shut up. I will shut up the shop at 5pm.
And now I’ll shut up. Your turn, what’s your favourite pair of non-opposites?