Yes, it is very offensive expression. 찌질하다 means "very timid, stupid, foolish, ugly, selfish, not very smart, not very tidy, not very clean....." it is really hard you know, if I rate the word, it would be 8~9? (or 10 for young generation I think.)
8 for this : 너 찌질하게 왜 그래?
10 for this : 아, 저 찌질한 놈