Persistent swearing (as a witness) to the truth of a particular statement, regardless of strong claims or evidence as to its falsity; (hence euphemism) perjury.
Origin
Mid 18th century.
Definition of hard swearing in US English:
hard swearing
noun
Law historical
Persistent swearing (as a witness) to the truth of a particular statement, regardless of strong claims or evidence as to its falsity; (hence euphemism) perjury.