What is the meaning of Fallout?

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.

Source: wiktionary.org