The Collaborative International Dictionary of English v.0.48:

Diatribe \Di"a*tribe\ (?; 277), n. [L. diatriba a learned
   discussion, Gr. ?, prop., a wearing away of time, fr. ? to
   rub away, spend time; dia` through + ? to rub: cf. L. terere,
   F. trite: cf. F. diatribe.]
   A prolonged or exhaustive discussion; especially, an
   acrimonious or invective harangue; a strain of abusive or
   railing language; a philippic.
   [1913 Webster]

         The ephemeral diatribe of a faction.     --John Morley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diatribe
    n 1: thunderous verbal attack [syn: fulmination, diatribe]