1. noun. A long piece of webbing, hanging loosely between two solid objects but not touching the ground. Used to walk upon while attempting to maintain balance for entertainment, usually drunk, usually by climbers. Advanced slackliners are able to: reverse direction, stay on indefinitely, jump, juggle, swing to and fro.
2. verb. To play on the slack line.
Those hosers think they're setting up a slack line, but it's more like a tightrope.
Dude, you gotta have at least another beer before you stop falling off the slack line.