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!