I love figuring out lyrics and stuff like that. One particular chorus that has always made me wonder, is from "What a Good Boy" by the Barenaked Ladies:

“This name is the hairshirt I wear
And this hairshirt is woven from your brown hair
This song is the cross that I bear
Bear it with me, bear with me, bear with me”

After reading it I would always kind of stop and think, “hmm, the hairshirt thing is a little weird.”
In a totally different topic - I started reading The Da Vinci Code by Dan Brown the other day. This book is the first book I have read since reading "Christine" by Stephen King, that actually makes me so scared at night, that I lay frozen in my bed. Its a good thing I wear diapers. Moving on. The author writes about the Opus Dei sect of the Catholic Church, among other things. Apparently people of that sect are required to wear a spiked belt (called a cilice) around their thigh for about two hours a day to remind them of the pain Christ suffered for all of us heathens. I find this fascinating. So, this morning I was dinking around the Internet in search of a picture of the device. (I have yet to find a picture,) but I did find out that in addition to cilices; hairshirt’s are another kind of bodily mortification used by Catholics. The hairshirt was made of Goats hair in hopes to aid the wearer in avoiding temptations of the flesh. Seeing the word hairshirt made me think of the song by BNL. Also – just as a side note – REM wrote a song called hairshirt, on the album Green.
So I think BNL is trying to say is that the girl they are singing about is so awesome that they are willing to torture themselves in honor of her. That is the short hand version for now.