To flail wildly and absurdly at the air with one's back leg as one's owner attempts to "help" by scratching an itch which, in all honesty, you had perfectly well covered. Some say the air scratch is not as involuntary as it seems, and is actually an obscene gesture whose roots stretch back as far as ancient egypt: a kind of feline version of a two-fingered salute, but much, much ruder. Others just decry it as another forlorn symbol of man's increasing interference in cat culture, a debasement of nature that will ultimately send us on a road to a dark place where a word like "natural" no longer even has any meaning.
Setting A Pissident
To urinate in a completely new and innovative place, instigating a trend for such action amongst your fellow felines. Born leaders but also generally kind of snotty, cats who set a pissident know that their originality comes at a price, and, upon seeing others follow lamely in their wake, can often be heard to mutter comments like "here come the mindless vultures, picking over the corpse of my brilliance" and "now I know how The Beatles must have felt when they heard the Marmalade's cover of Ob-La-Di, Ob-La-Da."