The Collaborative International Dictionary of English v.0.48:

guesstimate \guess"ti*mate\ n. [from guess and estimate]
   an estimate based on little information, being little better
   than a guess.
   [PJC]

WordNet (r) 3.0 (2006):

guesstimate
    n 1: an estimate that combines reasoning with guessing [syn:
         guesstimate, guestimate]
    v 1: estimate based on a calculation