They say it's really bad... （かす）

He Says:
kasu　（かす）=dreg,scum
This is a very dirty word to insult someone. Because it's dreg!
You can combine this words with other dirty words to be dirtier. It's like a punch,hook,uppercut!
So maybe Japanese girls will cry or end the relationship with you to be said this dirtiest word!

She Says:
Kasu comes from the scum that floats to the top when making sake or tofu. It's gross and unnecessary. Now, of course, people would use it as an insult, towards someone they think is beneath them. I'd translate this more as scum than dreg. We don't use the word dreg so much.

The next word is...ちんかす。