Acronym for what may be euphemistically called 'Within A Fly's S___ [Excrement] of Just', a term coined by Dave Keenan to designate a non-just non-equal tempered scale whose tones deviate from the just ratios of a specific just intonation system in such a way that the deviations are kept extremely small, typically about 3 cents or less. If the deviations are slightly greater, about 3 to 8 cents, it is referred to as quasi-just rather than wafso-just.
The goal of the tempering is to have the smallest set of pitches represent the largest number of just intonation harmonic relationships. Keenan has typically derived fairly extended 7-limit and 11-limit implied systems with scales consisting of 12 tones. Several others have been constructed by Carl Lumma.
Mainly because of the etymology of "wafso", Dave Keenan now prefers to use the term microtemperament.
More on 'wafso' may be found by searching the archive of the Yahoo Tuning List.