Kurwa

Kurwa is an exclamation done by Polandball when he gets mad or is worried. Polandball often use it when Germanyball and Russiaball try anschlussing him.

This word means whore or f**k

Poland
He uses this word the most, since it's from his language

Czech, Slovakia
They use a variation of the world known as "Kurva". You don't see it a lot because neither of them 2 appear in a lot of comics.

Croatia
Perhaps the most infamous, Croatia hates kebab not because he wants to follow brother Serbia, but because the arabs call him "Kurwa-tia", not very pleasant, I'd agree. He gets quite agitated when called so.

Taiwan
Coincidentally,Taiwanball's traditional language, Taiwanese, has a swear word that sounds similar to "Kurwa", which is "靠夭". The literal translation is "You're crying because your hungry". Weird.