The Collaborative International Dictionary of English v.0.48:

Extemporize \Ex*tem"po*rize\, v. i. [imp. & p. p.
   Extemporized; p. pr. & vb. n. Extemporizing.]
   To speak extempore; especially, to discourse without special
   preparation; to make an offhand address.
   [1913 Webster]