The story
The name is given because it was used as insecticide in England, similarly to many other European countries, compare e.g. Swedish flugsvamp and Russian мухомо́р (muxomór).
Etymology from Wiktionary, CC BY-SA 4.0.
fly agaric
Today it means: A poisonous and hallucinogenic mushroom, Amanita muscaria, with a large crimson cap, fading to yellow, scattered with white flecks. Full definition →