Just back from my shodō (書道) class, where poor Watanabe-sensei keeps trying to teach a bunch of hyperactive, loud-mouthed Argentinians to write nice kanji and insists on using an old Chinese style just when I had more or less mastered kaisho.
The good thing about Watanabe-sensei is that she knows precious few Spanish words, so I'm forced to practise my Japanese. Moreover, I'm by far the most advanced Japanese student in her class (which is not to say I'm fluent or even coherent), so I also have to translate what the others want me to tell her. Usually we end up resorting to the denshijisho (electronic dictionary) that she carries around. Today I learned two new verbs, which I'll have to put in use soon so as not to forget: suberu "slide, glide, slip" and kaku "scratch".
Try this: Shiri o kaku koto wa shitsurei desu. "It is a discourtesy to scratch [your] ass." There's really no "your" in there, but we can safely assume you won't often be scratching anyone else's ass. Change desu to da if you're talking to friends or family; change it to de gozaimasu and say oshiri instead of shiri if you're being extra-polite (for example, if it's a customer or a venerable old lady/gentleman who's scratching her/his ass). Throw in sumimasen and sukoshi if you feel you might come out as too blunt. It's a matter of people's asses, after all.
Then, with the other verb, picture the following: Ojiisan wa banana no kawa de subete shimatte, minna ni warawareta. "The old man slipped on a banana skin and got laughed at by everybody." The example is a bit graphic but hey, it's my study method.
(Just read the first example and found out a mistake. The grammar seems right, but it really should be Hoka no hito no mae de shiri o kaku koto wa shitsurei desu, "It is a discourtesy to scratch [your] ass in front of other people." What was I thinking?)