An Etymological Dictionary of the German Language/Baas
Baas, m., ‘master,’ a LG. word; comp. Du. baas; orig. perhaps it was a term of endearment used in addressing superiors (comp. Amme, Muhme, Bube, Buhle). It is undoubtedly connected with Base, ‘aunt on the father's side,’ because Base, Baas — Bäsel, are also titles given by domestics to their mistress. Yet it is astonishing that the area of diffusion of Baas, m. (LG.), and Base, f. (MidG. and UpG.), is different. Perhaps ‘paternal’ was the root idea of both words.