Uncle Vernon didn't go to work that day. He stayed at home and nailed up the mail slot. "See" he explained to Aunt Petunia through a mouthful of nails, "if they can't deliver them they'll just give up." "I'm not sure that'll work, Vernon"
It's from Harry potter, and I'm not sure if 'mouthful of nails' here means 'Uncle Vernon's mouth was full of nails' or.. Does it have something completely different meaning? Cause I thought it's strange if someone is stuffing his mouth with nails.