A monk asked Ummon, "What is Buddha?" Ummon replied, "Kanshiketsu!*"
* shit stick - what people back then used as toilet paper
Ummon also appears in Koan 15 ('Tozan's Sixty Blows') and 16 ('When the Bell Sounds'). He seems to be a rather colorful and feisty guy, which may explain why he had such an incredible reputation as a teacher and how he was able to found one of the five schools of Zen.
Anyway, this koan is very much like Koan 18 (Tozan's 'Masagin'). In my entry about that koan, I had mentioned that I'd thought I'd heard a nastier version of 'Masagin'. This koan must be the one I was thinking about!
Koan 18 and 21 are exactly the same, other than the name of the monk who is asked about Buddha and the answer he offers, so the basic interpretations I have are the same. Beyond that, the focus in 'Masagin' may have also been on the possibility of it being easier to have direct perception with something like masagin - or some other mundane object. In 'Kanshiketsu' Ummon's...er...earthier answer suggests something a bit different to me on top of the basic interpretation.
In a good deal of modern Zen writings of the new age flavor, there's stress placed on 'inner peace' and 'oneness' and 'world peace'. There's also tons of accoutrements you can buy at stores that get into that line of thinking: Zen gardens, clothing, pillows, music, gongs, crystals, incense, statues, self-help books, vegan recipes, etc. By saying Buddha is something you wipe your ass with, Ummon reminds us of the raw simplicity of Zen. This is what has led me to avoid - as much as I can manage to do so - the purchase of 'Zen stuff' or adopting 'a Zen lifestyle or diet'. I feel like it's the absolutely wrong way of going about it, innocent as the purchases and intentions may be. It also makes me not avoid necessarily but shy away from talking about Zen too much with people.
Maybe if I think 'shit stick' every time I'm tempted in the wrong direction, then I can completely avoid it altogether!