¿Qué significa "castigo"?
I can't tell if English or Spanish is your native tongue so I will answer in English. It means to punish. The English equivalent is "castigate" which of course means to punish. Castigar is a great word because it immediately relates to it's English counterpart.
My dad is a lawyer and uses the word "castigate" quite a lot.
Edit: the reason I didn't immediately use the word cognate is because castigar falls into a sort of interesting category of cognates that relate differentely to their respective counterparts. An example of a well known cognate is "producir" to produce or "deducir" to deduce. There are other cognates however which seem to be a bit different to me. "Castigar" - to castigate - to punish. "Masticar" - to masticate - to chew. "Comenzar" - to commence - to begin. "Picante" - piquant - spicy.
There are what I think of as either "super cognates" or "third cognates" because instead of directly relating there is a process where they must be translated again. Sure tranquilo means tranquil but it really means calm. Those words are similar but they are slightly different. Culpable means culpable but it really means guilty. You catch the drift. I consider these cognates to be slightly different than the typical cognate. But then again I not a native speaker so I'm just throwing my educated opinion around.