Brisingamen was treated as a proper noun in the book title but only Brisinga is. The Brisingar are the dwarfs who made Freyja's famous necklace. 'Men' is Old Norse for 'necklace'. I wonder if Garner knew that when he picked it for a title. There is no necklace in the story but there is a stone. It has little or no connection with wyrd either, which was the Old English word for 'fate' or 'Fortuna' but then Garner chose the later spelling of 'weird'. So we are probably to take it as 'odd' or 'strange' but it looks odd and strange to me.