a trick done using the smoke from either a cigarette, or a joint. One inhales through the mouth, exhales most of the smoke, but inhaling the exhalation through the nose, and finally exhaling everything through the mouth again. It's a bit tricky, so you might want to practice before trying to show off your skills in public, thus making a fool of yourself.
Man, I've been trying to french inhale forever, and finally got it down. My sinuses are wrecked!