Kasna literally means to tighten or strengthen. To say "rassi ko kass kay bandho" means "tie the rope tightly". Another way to use it would be "kas kay maro" or "hit/beat hard" or "hit/beat with force". In the sentence you've used, it is more idiomatic than a literal usage. So, "jumle kasna" would be similar to berating someone or verbally insulting/being harsh with someone.