[zimdog]

That phrase has been around for ever and isn't derogatory.

Of course it's derogatory, you mush head! It's an insult!

[commonguymd]

I hear Rush saying it in quite a delightful way all the time. A "skull full of mush" describes youths' minds that are to be molded and shaped during the learning process at school, home and church. I am far from youth, but perhaps you are being sarcastic. So many are so quick to take offense to everything.