Slijmbal literally translates to suck up. Someone who tries to get into someone’s good graces. I think the main reason is that he couldn’t find a proper acronym for Uil, so he had some fun with it.

S.L.IJ.M.B.A.L. stands for ‘Schriftelijke Loftuiting wegens IJver, Magische Bekwaamheid en Algeheel Leervermogen’, a diploma in the Harry Potter series.

It means Written Praise for Diligence, Magical Skill and General Studiousness.