Verb. The heavy metal equivalent of humming or whistling to oneself, in which the sound of a guitar riff is simulated with the mouth. Onomatopoeia for the sound of an open power chord inserted between furious bouts of "palm muted" 32nds. The term can be applied to any style of heavy metal mouth-riffing, even when the "dash" sound is not the most appropriate for the riff in question.
For illustration, Slayer's "Angel of Death" (main verse)
dash-shshshshshsh dash-shshshshshsh dash-shshshshshsh da na naa (repeat)
Johnny: *dashes absent-mindedly*
Sean: Hey do you know you sound completely retarded
Johnny: Ah shit. There's nothing more embarrassing than being caught dashing.
Kid (listening to Slipknot on iphone): "Man, this is awesome! da-"
Person 2 (wearing Morbid Angel sweatpants): "Hold it right there. Dashing may only be performed by the disciples of true metal."