A WAG (wild-ass guess) is an off-the-cuff estimate for something, such as the completion date of a project, when there is insufficient data available to support a more informed estimation. In Agile and scrum software development, a WAG is often based upon historical experience.
Because WAGS are simply guesses, they can be very difficult to substantiate. A scientific wild-ass guess (SWAG) is a WAG that is explained in terms of assumptions and informal calculations made to arrive at the estimate. For example, coming up with a WAG for a project completion date, a project manager could explain that a similar completed project took six months but also that there are fewer developers available currently, so the project might take an extra month. The scientific part of the SWAG is the additional explanation and calculation.
Planning poker, a team building activity for achieving group consensus, is a popular tool for creating WAGs that are more likely to be realistic.