"Gankshank" combines the words "gank" (a gaming term meaning to ambush or betray, steal from someone, usually with the help of other teammates) and shank (a slash or stab with a knife). To "gankshank" is to ambush someone and stab them. Unexpected and nefarious shankage.
A criminal-minded person who commits or has committed an armed mugging and murder with a knife or knife-like weapon. (i.e.- the bastard ganked your wallet and shanked your sorry ass.)
As described by Mike Doughty (Soul Coughing) in an interview publishedOctober 16, 1998: "a kind of guitar rhythm I invented in an attempt to recreate a hip-hop groove on an acoustic guitar--it goes gankadank, gank-duh-did-dank."