champignon (n.)

1570s, "a mushroom," from Middle French champignon (14c.), with change of suffix, from Old French champegnuel, from Vulgar Latin *campaniolus "that which grows in the field," from Late Latin campaneus "pertaining to the fields," from campania "level country" (see campaign (n.)).

The French word for mushrooms generally; in English the sense gradually narrowed 18c. to the edible species, and especially that growing in fairy rings.