Hip hip huzzah! I've been waiting for someone to start this thread so I could crow about how smart I was in creating my screen name.
"fordim" is an archaic verb that means to obscure by casting into darkness (one fordims the living room couch by turning off the lamp)
"hedgethistle," well, OK isn't so smart as it's just a compound of (obviously) 'hedge' (in the sense of a bordering hedge meant to mark the boundary of a property for protection: cf the hedge between the Shire and the Old Forest) and 'thistle'.
So now that I look at it, I guess it's not that smart at all.
But my SN means "an obscuring protective hedge composed of thistles" -- it seemed appropriate insofar as it's meant to be a mask behind which the 'real' me (whoever that is) can hide.