A game jam is like a weekend hacking event where a bunch of people make small games in a short time frame and then get feedback or compared. A demake is a remake but usually for older systems or in an older style / lower resolution. In the indie game scene lots of the big hit games were first created at a game jam. No tomfoolery, they're useful words to describe things, that don't have other easy ways to describe.
Demakes can also be games that boil down the essence of their genre or original game to simpler rules. A great example of this is Footsies, which is a demake of the trad fighting genre.
