The act of dancing with one finger (generally the index finger) extended. Can be done with one or both hands.
Disco finger has existed as long as mankind; in ancient Egyptian hieroglyphics we see the pharaohs, digits extended in exaltation of what can only be described as the precursors of pop.
The are many variations on the current disco finger--arm-pumping, twirling, the doorbell--and possible adaptations are still being discovered to this day.
While the rest of the crowd engaged in the ubiquitous "rock fist," Benway stood out from the crowd with his enthusiastic disco finger, thus showing his appreciation of the music.
When a girl or guy sticks his or her finger(s) up a girl in a bar or nightclub.
I gave Sarah the disco finger last night. She loved it!