He and Andrew were both described as incompetent; this was particularly evident when Jack knocked himself unconscious with his own bat. He expressed interest in continuing to play for Gryffindor, but was replaced the following year.
Jack derives from John, which means "God (Herbrew) is gracious".
Sloper could come from the English word slope,a surface in which one side is higher than the other. This could relate to his inability to do things right, since slopes have been described as very hard to climb for some people.