Skank is the sound that a guitar or keyboard makes in reggae music. That steady upbeat chop that is perhaps the most definitive sound of reggae. The skank was invented by the Skatalites as a way to make a unique sound to Jamaican music. It has lasted from the early 60's ska days until present day.
Keyboard players also can do a variation on the skank that is called the bubble. The bubble is kind of a triplet skank.
It is interesting to note that the mandolin part in much bluegrass has the same feel as the reggae skank.