A Geddicorn is a mythical creature similar to a unicorn. It is a good-looking female that attends a RUSH concert without having to be coaxed to do so by a dude.
She is turned on by Geddy Lee and not afraid to show it.
I went to the RUSH show last night and saw the hottest Geddicorn ever. She was in the 4th row on Geddy's side and wasn't even with a dude!