I am just a man in a silly red sheet

I was surfing the web earlier and came across the phrase “I am just a man in a silly red sheet” in more than a few blogs.
Some people changed it to read, “I am just a girl in a silly red sheet.” Nevertheless, the ‘red sheet’ thingy was confounding me.
I first noticed it on my brother’s webpage and alerted him to it. I thought he had a typo and meant to say ‘silly red shirt.’
I decided to see what this meant and came up with some stuff I was previously ignorant about.
For example it is part of the lyrics of a song named ‘Superman’ and in fact, has nothing to do with Superman the movie!
Turns out I have heard this song many times before, but have not really listened to it.
The next time I hear it, I will be sure to listen.