On the Discworld, the number between seven and nine is considered bad luck. Nobody says it by name, for the same reason you don’t say things that might get your soul eaten by the dark god Bel-Shamharoth. (That reason being, of course, that it might get your soul eaten by the dark god Bel-Shamharoth.) For this reason, people call it “7a.”
Rincewind is a professional runner-away-from of scary things. It’s a known fact that the universe is perpetually out to almost get him. He wasn’t surprised that he happened to live in a room numbered 7a.
My apartment? Floor eight, room 7a.