it must be 靠（kao） 操（cao）

傻逼；白痴；。。。

It depends on the language. Chinese isn't really a language and more of an container term for various different languages - or the written form. I don't curse, so my knowledge is limited. 250 is used in the north, so is 傻冒. Others can give you more input on that. In Cantonese by far the most common curse term is 屌你 or 屌你老母. For some people it's basically in every sentence they speak.