in a strict sense, when a bassist tackles a singer onstage with their bass still on.
in a general sense, when a (usually introverted) person does something out of character, whether appropriate or not. usually a prank that involves physical contact of some sort in which the inductee gets mad.
derived from a Dream Theater show, where the normally quiet bassist John Myung tackled vocalist James LaBrie, to the amazement and amusement of the crowd. it was later revealed that someone bet him a couple hundred bucks that he wouldnt do it.
dude1:Did you see that concert where John takcled James?
dude2:hells yes. that was hilarious.
That was so hilarious when Mike pissed in Jordan's beer. That was definitely a Myung tackle, because Mike refuses to talk to Jordan to this day.