Source : Webster's Revised Unabridged Dictionary (1913)

Prestissimo \Pres*tis"si*mo\, adv. [It., superl. of presto.]
   (Mus.)
   Very quickly; with great rapidity.

Source : WordNet®

prestissimo
     adj : (of tempo) as fast as possible
     adv : extremely fast; as fast as possible; "this passage should be
           played prestissimo"