The way stupid people say "estimate". They either think they're being cute or clever, or else that it's an actual word. Since it's not any of these, science has yet to offer an explanation for people who consider use of this word acceptable.
My college trig teacher asked us to "guesstimate" values. I wanted to punch her in the mouth.
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)