is the correct answer. A "mac" is a colloquialism for a type of raincoat (see below), Arthur is Ron Weasley's father in the Harry Potter books.
(Incidentally, I was told this one many years ago:
Q. What do you call a man in a raincoat? A. Mac.
Q. What do you call two men in raincoats? A. Max.
Q. What do you call two men in raincoats standing at the gates of a cemetery? A. Max Bygraves.)
And it's back to Sonofagunzel.