cursor dipped in X n.

There are a couple of metaphors in English of the form `pen dipped in X' (perhaps the most common values of X are `acid', `bile', and `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!"

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.