Noun
The false, anti-Semitic accusation that Jews abduct and murder non-Jewish children to use their blood for their religious rituals and holidays.
(by extension) Any false or purportedly false accusation of murder of a non-Jew by a Jewish person or organization.
(by extension) Any false or purportedly false accusation of guilt, especially of guilt in mass murder or homicide.
More generally, any canard or virulent lie about someone.
Source: en.wiktionary.orgA man holding a "Blood Libel" sign during a heated Board of Health circumcision hearing in 2013. Source: Internet