It's a childish way of saying this, but I think it's common enough to add it.
I am confused. Does this mean 'all of you open your one collective mouth' or 'all of you open your (personal) mouths'?
I am not sure if I understand you but "usta" if singular and plural form, depending on context. Like "drzwi", "spodnie", "nożyczki".
The English translation needs to be "open your mouths" as "otwórzcie" refers to multiple people. For one person, it would just be "otwórz usta", wouldn't it?
You're right. I'll remove any plural options, as we are translating singular "mouth" here.
why here not accusative
It is. But "usta" are 'not masculine-personal plural', so the Accusative form is identical to Nominative.
So usta is pluralis tantum and usto simply doesn't exist (like the non-existence of a singular form of drzwi)?
Pretty rude. Interesting that you're the second person to ask that recently...
What would that translate to English? :)
Hmmm... Wiktionary says "gob": https://en.wiktionary.org/wiki/g%C4%99ba