A very bad theory about theories

In some venues, this is a familiar trope: a science advocate responds to the criticism that “evolution is just a theory,” explaining that, for scientists, a “theory” is a thoroughly tested, well established explanation.   Yet, for over 200 years, scientists have used “theory” for conjectures that are not well established, even ones whose truth is rejected, e.g., Lamarck’s theory.   What is going on?