According to
August 7, 2001's
A.Word.A.Day's mailing, vexatious can also mean a
legal action with insufficient
legal basis which is filed purely to annoy the
defendant.
The etymology of vexatious is fairly straight-forward, as oftentimes muddled English words go. Derives from Middle English vexacioun, from Latin vexation, from vexare, to vex or annoy.