Gobbing is the practice of audience members spitting on a band as a form of applause.
Gobbing is a fine tradition of punk music, and more specifically the UK punk music scene in the 70s and 80s. Legend has it that the practice started when Rat Scabies from the band Damned objected to an audience member throwing a beer at him, and spit in his face in response. There are also reports of Damned attending other band's gigs and spitting on them from the mosh pit as a sign of disapproval. Johnny Rotten, on the other hand, believes that he started the trend, as he had a habit of spitting on stage -- not at anyone, just onto the stage.
Gobbing wasn't just an occasional gob, either. Popular bands would be greeted with what looked like a hail storm of spit, and reports of this era include phrases like "...couldn't tell where their dreads ended and the spit began." Band members report the amount of spit making guitar strings slick and causing instruments to rust. There were also health risks -- Siouxsie reported catching conjunctivitis after spit hit her in the eye, Joe Strummer caught mono (many sources say hepatitis) after some landed in his mouth.
Reportedly, gobbing still carries on today, although it is certainly not what it was in its heyday.