Generally, a guitar player of moderate skill who refuses to accept the limits of their abilities, and as a result often embarrases themselves while playing. However, the guitar hack does not realize how much they are embarrasing themselves, and rarely makes an attempt to improve their guitar skills.
A guitar hack can often be recognized easily, as they will usually own a rather expensive electric guitar, such as a Gibson Les Paul or an upscale Fender, and still be able to competently play only the most basic 12-bar blues and other simple songs.
In addition, some guitar hacks are decent players in all but one regard, these are usually refered to as rhythm hacks and lead hacks.
The author of this writeup is by all accounts a rhythm hack.