Did you know that bumf is an abbreviation of bumfodder. In Brit-speak, your backside is your bum.
Paper in a form specifically designed for the use of wiping one’s bits clean after defecation wasn’t introduced until the middle of the 19th century, but waste paper was commonly used for that purpose much earlier. In the 18th century it was apparently commonplace to tear pages out of cheap books to use as lavatory tissue, and it appears some people would buy books both to read when on the job and for cleaning up afterwards. (Try doing that with a lap-top.)
This practice gave rise to the word bumfodder, which the Oxford English Dictionary defines as: Toilet paper. Also occasionally: a piece of this. Attributive and allusively. Worthless or inferior literature; any written or printed material that is perceived as useless, tedious, or unnecessary.