Context : CriticalRationalism
Need to write this properly ...
but basic idea is that for Popper, "rationality" is not so much a "property" that agents have but a propensity or something realized in action and existence (the capacity / willingness to change beliefs based on new evidence.)
It's closer to those existentialist qualities such as as "authenticity" or "grace" or "freedom" than it is to a thing that is a "given" for you.
See also :