1. Originally used by morons who like to make up words, guesstimate is a combination of two words that don't mean quite the same thing; A combination of the words Guess and Estimate that has adopted the meaning of the word guess.
2. To Guess; To draw a conclusion with little or no reason to believe so.
Bob: How high are we on this building?
Mark: I'd say about 10 stories.
Bob: That sounds like a good guesstimate.
Mark: (Silent in awe of Bobs stupidity)