Ius Strictum means "strict law", or law interpreted without any modification and in its utmost rigor. It is a very rare term in the materials of classical Roman law

Roman law

Roman law is the legal system of ancient Rome, and the legal developments which occurred before the 7th century AD — when the Roman–Byzantine state adopted Greek as the language of government. The development of Roman law comprises more than a thousand years of jurisprudence — from the Twelve...

. It is really a Byzantine

Byzantine law

Byzantine Law was essentially a continuation of Roman Law with Christian influence, however, this is not to doubt its later influence on the western practice of jurisprudence...

term, occurring in Justinian’s Institutes in reference to the strict actions of the law, primarily describing the rigid limitations of the forms of action available under the law, particularly with older laws. It is often used by later commentators to distinguish it from the moderating influence of the praetors, or judges who expanded the law through actions ex fide bona, or what we would now call equity.