Drinking an entire bottle of cough syrup in order to get high. This is because the active ingredient in cough syrup is codeine, a morphine derivative.
Named after Robitussin, a brand of cough syrup.
WARNING: Causes seriously bad trips much more often than good.
Jack: Hey, those guys are doing tussin, let's join in.
Jill: No way man, I ad a fucking nasty experience last time I did that. I'd seriously rather eat shit.