Green’s Dictionary of Slang

1. stupid, dull.

1597

Pilgrimage to Parnassus ii 168: Woulde anie leaden Mydas, anie mossie patron, have his asses ears deified, let him [etc.] [OED].

2. (US) very conservative or reactionary; old-fashioned, old.

1932

L.W. Merryweather ‘Argot of an Orphans’ Home’ in AS VII:6 402: That’s a mossy hat he wears.