The metaphorical source of the electronic equivalent of a poisoned-pen letter. Derived from English metaphors of the form "pen dipped in X" (where X = e.g. "acid", "bile", "vitriol"). These map over neatly to this hackish usage (the cursor being what moves, leaving letters behind, when one is composing on-line).
"Talk about a nastygram! He must've had his cursor dipped in acid when he wrote that one!"