Arabic for
unbeliever,
heretic,
apostate,
pagan,
athiest,
infidel,
heathen, or any such word that has to do with not believing in
god.
Derived from the three letter root verb, Kaf Fa Ra, meaning to block, to cover. In pre Islamic Arabic poetry and writing, one would find passages like, "kafara (covered) al hofra (the hole)." In modern times, Kafar also mean car tyres because it blocks and cover the air in the tyre.
It almost sound like the English cover both in meaning and pronounciation!