Fallout meaning : The event of small airborne particles falling to the ground in significant quantities as a result of major industrial activity, volcano eruption, sandstorm, nuclear explosion, etc. The particles themselves. A negative side effect; an undesirable or unexpected consequence. A declined offer in a sales transaction when acceptance was presumed. The person who declines such an offer. An impromptu guest used to fill in for another guest spot who is a no show or who has cancelled last minute