Any of various dark-bodied terrestrial slugs; especially the large Arion ater (family Arionidae), a common slug of northern Europe and northwestern North America that is typically black-bodied.
Origin
Mid 18th century.
Definition of black slug in US English:
black slug
noun
Any of various dark-bodied terrestrial slugs; especially the large Arion ater (family Arionidae), a common slug of northern Europe and northwestern North America that is typically black-bodied.