It’s time for you to get your backside off the couch.
A question for muttersprachlers: does anyone say it like this? Failed to find any example of such idiom in the Internet. If yes, then which word is meant here exactly? According to the explanation by ChatGPT, it is about “Po”, but the word “Po” has only 2 letters, not 4…
Yes, you can say it like this, though from my own experience nowadays people would directly use (der) “Hintern” instead of being so vague about it.
“Es ist Zeit, dass du deinen Hintern (vom Sofa) bewegst.”
As for what the word with four letters is, I would have guessed (der) “Popo”.
I checked it online and it seems to be one of the possible explanations of what the 4 letter word is supposed to be, but it isn’t confirmed which word it came from.
For further reference:
https://en.wiktionary.org/wiki/vier_Buchstaben
Looking into it even more, some other natives seem to remember it from their childhood, which is also true for me. I’m not sure, if is because it’s only used towards children or also because it is out of vogue by now.