A term for someone who is not only a tool, but also a piece of shit. Hence the word 'stool' is used to describe them, transforming the word tool into a similar word describing fecal matter. Also, since a stool is also a tool for people to sit on, being viewed as implement of this nature would generally be seen as undesirable.
Guy 1: That dude is a total stool!
Guy 2: Yeah he's a tool and a turd!