A guestimation is an entirely useless term that resides between guess and estimation.
Dude 1: My guestimation is about $1,500 for that car.
Dude 2: ?
Half way between a guess and and an estimation.
Guy1: You were right! Edouard really is an idiot!
Guy2: So, my guestimations were correct! I knew it from the very beginning!