I recently learned about the term arak, which refers to a type of Middle Eastern liquor infused with aniseed. According to the Wikipedia article, the drink is named after the Arabic word araq, meaning "sweat".
The linguistic similarity between this word and "Arrakis" seems quite strong. Both the fact that this is a spice-flavored drink, and that wearing a stillsuit causes one to actually drink sweat, seem to indicate that Herbert had this term in mind when inventing the name of the planet.
I've only read Dune itself, which as far as I remember doesn't identify the origin of the planet's name; do any other books in the series--or Herbert's out-of-universe essays/interviews/etc.--back this up?