Stare/starci (+ slang)
In a more common Italian usage, starci (stare + ci) would be generally used as "to fit". e.g. Questa sedia è troppo piccola, non ci sto. = This chair is too small, I don't fit in it.e.g. Ci sono troppe valigie, non ci stanno nel bagagliaio! = There are too many suitcases, they don't fit in the trunk.
Another not very slang use is "starci male" where starci is basically the shorten version of "restarci" (to remain/be left), -> restarci male (to end up upset). e.g. Non starci male = Don't be upset about it./ Get over it./ It wasn't worth it.
Slowly growing into the slang scale, we can find the acceptions: "to be in the game", "to go along", "to agree". e.g. Se ci stai, direi di proseguire per questa strada = If you're in (the game)/If you agree, I'd suggest to keep walking on this road
And from there, the acceptions: "to agree", "my opinion/this thing fits this reality/situation" (only with the 3rd singular person: ci sta/non ci sta).... e.g. Ma ci sta che tu ti senta triste dopo tutto quello che ti è successo = But it's okay/right/it fits that you feel sad after all you've been through. The negative form is more rare in this 3rd person usage; we generally say it in a more personal way: e.g. (io) non ci sto = I don't agree. /I am not in the game. If we are not sure about something and we want to use the conditional (like we're taking that as a possibility, we're evaluating all the possile scenarios and the one you said can fit/we might agree to it), we can say: ci può stare =it can fit, it makes sense.
It could also be used when you're trying to flirt with someone, to say to your pal if that someone is in the game or not, or just when you comment with your friends about someone else's romantic situation: e.g. Lei non ci sta per nulla = She isn't interested (in them) at all/ she doesn't play along. e.g. Le/Gli ho chiesto di uscire ed ha accettato! Dai che ci sta!! = I asked her/him out and s/he said yes! Yay s/he's interested!!
Now let's suppose our friend is upset cause something that nobody can do anything about happened to them (maybe their partner left them? Idk first example that came across my mind, but could be whatever really, even that their car broke). If we were a lot into slang Italian, we could say: "stacci". = allow that to be, accept it, let it fit you, let it flow. Go with the flow. (Romans would probably say it as "stacce", and use it much more than us, I think).
Another slang sentence that I want to share with you is "starci sotto" = to be under. You can (at least you were able to) find it or hear it also as "stare sotto un treno per qualcun*" = to be under a train for sb; it means you have a very huge crush for someone (you could say also "sotto mille treni" = under a thousand of trains, if the feeling is very strong). From this saying originated the famous noun "sottone/sottona" as to refer to someone who is a very submissive partner. I found "simp" as an English translation, but honestly... I feel like it's lightly different? A sotton* is someone who basically turns into a carpet for the other person, and do whatever they ask them to do without even noticing how poorly they are being treated. Like having heart eyes but in all your body?? I hope you got it.
Beware: do not confuse sottona with sottana, the latter one means skirt (colloquial, also used to refer to women in specific situations), slip, robe.
