An onomatopoeia used by DFW in Infinite Jest within the context of tennis matches. This definition from AllExperts is accurate and complete:
A term used in the novel Infinite Jest roughly meaning to insult or mess with someone. It can be used as a noun, e.g. Bobby felt his third nipple was an existential kertwang. Or a verb, e.g. God enjoyed kertwanging Bobby by sending gusts of wind to blow up his shirt to reveal his third nipple.
The word is onomatopoeia because of its usage in conversation regarding tennis matches, with opponents rushing the net, "kertwanging," giving the sense of the sound of the strings in the tennis racket.