You often wonder why, of all the known words in the dictionary, we call a hack, a hack. Well, it seems that whosoever first called the process of breaking into a system a hack was aware of this 125-year-old letter, where the word first appeared.
When someone hacks into your account or your computer, your first reaction is of trepidation; everything else comes later. So, a hack has a negative connotation, although there are ethical hackers in our midst, too.
To have a detailed understanding, you may read this article on Wired.