ROTFL

The negative form of can1.
 
Usage Note: The idiomatic phrase cannot but has sometimes been criticized as a double negative, perhaps because it has been confused with can but. The but of cannot but, however, means “except,” as it does in phrases such as no one but, while the but of can but has the sense only, as it does in the sentence We had but a single bullet left. Both cannot but and can but are established as standard expressions. ·The construction cannot help is used with a present participle to roughly the same effect as cannot but in a sentence such as We cannot help admiring his courage. This construction usually implies that a person is unable to affect an outcome normally under his or her control. Thus, saying We could not help laughing at such a remark would imply that one could not suppress one's laughter. ·The construction cannot help but probably arose as a blend of cannot help and cannot but; it has the meaning of the first and the syntax of the second: We cannot help but admire his courage. The construction has sometimes been criticized as a redundancy, but it has been around for more than a century and appears in the writing of many distinguished authors. ·The expression cannot (or can't) seem to has occasionally been criticized as illogical, and so it is. Brian can't seem to get angry does not mean “Brian is incapable of appearing to get angry,” as its syntax would seem to dictate; rather, it means “Brian appears to be unable to get angry.” But the idiom serves a useful purpose, since the syntax of English does not allow a logical equivalent like Brian seems to cannot get angry; and the cannot seem to construction is so widely used that it would be pedantic to object to it. See Usage Note at but. See Usage Note at help.

So you cannot seem to understand it? Or you are incapable of appearing to understand? Or you cannot be able to understand it? Dang, I'm confused now.