Scientific wild-ass guess (SWAG) is an American English slang term meaning a rough estimate made by an expert in the field, based on experience and intuition.
[1] The slang term "SWAG" is generally thought to have originated in the US military, either the Army[2] or the Air Force.
[3] Journalist Melvin J. Lasky wrote that it was first used casually by US Army General William Westmoreland during the Vietnam War.
[4] An earlier use of term is mentioned by linguist J. Robert Dumouchel who wrote the 1975 book, Dictionary of Development Terminology.
Describing the factors that led to President Bill Clinton relying on the US Air Force in Kosovo instead of employing "200,000 NATO troops",[5] Harris wrote that NATO's estimate requiring 200,000 ground troops was regarded by American war planners as politically impossible to achieve; it was quickly removed from consideration.