Interesting question that kept me thinking. As far as I can see the term (as applied to laws, norms, or constitutions) appears prominentely first in the book "Der Hüter der Verfassung" (1929) (transl. as "The Guardian of the Constitution") by the influential German legal scholar Carl Schmitt. It's about who is responsible for the "defense" of the Weimar constitution: the president of the Republic himself or a constitutional court. But obviously, the term and concepot ist far older and appears, e.g., in pre-Justinian Roman law requiring guardians in the form of tutors to be installed for children.