So if someone says "Mikä keissi!" it can mean:
A case of beer
A beer belly
Or basically a case, like when there's this case and this case. How do you explain this word... Like police has a case - a murder case. Yes.
And now you wonder what "keissi" actually means? Well, basically it means "case". In real Finnish you don't call a pack of beer or a beer belly "keissi", but somehow slang has shaped the word in this form.