a highly toxic, yellowish, powdery substance found in Southeast Asia c. 1975-85, alleged by some to be a chemical warfare agent dropped from airplanes and by others to be contaminated bee excrement
See yellow rain in American Heritage Dictionary 4
noun
A powdery, poisonous, yellow substance reported as dropping from the air in southeast Asia and found to be the excrement of wild honeybees contaminated by a fungal toxin.