A guesstimate is a number you give when you do not really have the facts, so you guess and estimate at the same time. It is rougher than a careful estimate, but it is not a wild shot in the dark either — you are using a little knowledge and a little guessing. People use it as a noun (a guesstimate) and as a verb (to guesstimate). It is informal and a bit playful; the blend is meant to sound honest about how shaky the number is. You might hear it among friends, in offices, in planning meetings. In a school paper or a serious report, people usually write estimate and then say it is rough.