If you make a circle with your left hand and slam down on it with your right palm, it symbolizes passive anal sex as in "I fuck your ass." Similarly, the "OK" sign in english, if turned upside down, means "asshole," especially to Turks from the Southeast.
You didn't ask, but just as an FYI, the word "um" in english, as in the noise we make when we're thinking, means "pussy" in Turkish, so if you are looking for a uniquely crude gesture, just think loudly.
