SWAG is an acronym meaning “Sophisticated Wild Ass Guess”. It is used in the military world as well as in the software development discipline. It basically means that there is not enough time or information to deliver an exact estimate of what is needed, and as a consequence, an estimate is made based on what is available, be it part of the required information, be it nothing.
The term is sometimes also used as a verb, e.g. “The developer SWAGed the effort.” The term is mainly used in the US, and it is not an official PMI term. In the project management world, a SWAG may not be better than nothing since a project manager must make sure that reliable information is available for an effort estimate; otherwise, the project plan may not be worth anything. Even if part of the project requires research with unclear outcome, there are better approaches than SWAGs. One possible approach in such a scenario in project management is the use of Three Point Estimates.
Another term used for shaky estimates is “guesstimate”, a mixture of “guess” and “estimate”.