An
approximation based on some
amount of
known information. Commonly used in
estimates for
duration,
cost, or
difficulty of
technological projects.
This is NOT simply a made-up quantity. That would be a guess. Nor is it an educated guess. That concept differs from guesstimate by its lack of plausibility and/or accuracy. For some reason, appending "-timate" to "guess" is more reassuring than "guess" alone, or even coupled with "educated".
I'm confused by this rather sprained logic, but I guesstimate that it will take another few years before the definitions acquire a semblance of rationality.