Guesstimate

Guesstimate is a fancy word to define a bold estimate, an estimate made without enough details, knowledge and/or time.
Real world application, when somebody (usually your boss) is pushing hard to get an estimation and doesn't accept the canonical answer (need time to think about it, can't answer without some previous checks, ....) and you finally dare to answer, then the answer must be clearly a guesstimate.

Hope you don't need it a lot!