It's MODification, "My Own Design" is silly.
Also used quite extensively outside of the LEGO community, e.g. in gaming a MOD is altered game code you execute, either to cheat or too change graphics for fun.
So an existing set where you altered things to is a MOD.
A MOC is created from scratch.
According to BrickLink, the cheese slope, part number 54200, first appeared in a handful of sets in 2004.
In the mid-1990s, the AFOL community hung out on the Usenet newsgroup rec.toys.lego and its predecessor, alt.toys.lego. A Google Groups search of the archives of these newsgroups finds no references to the cheese slope.
At some point in the late 1990s, ...