Mr. Tumnus is yet another word for the Perenium (bonch, gooch, taint, grundle, chode, etc).
The use of this narnia name came from scroTUM, and aNUS, hence referring to your chode as Mr. Tumnus. Giving that special area of your body a name just makes it a little more personal. Kinda like naming your junk.
Mr. Tumnus is feeling a little cranky today, I should have wiped better after my dump last night.