Old-fashioned, unoriginal

I'm sure he has been a very famous psychology professor, but now his ideas are old hat and nobody will listen to him anymore

Sarà stato anche un luminare di psicologia, ma ormai le sue idee sono sorpassate e nessuno lo ascolta più

My boyfriend's mother is a very nice lady, but her ideas about man-woman relationship are a bit old hat for me

La madre del mio ragazzo è una cara e dolce signora, ma per i miei gusti ha idee un po' troppo antiquate sulle moderne relazioni tra uomo e donna

The expression dates from the late 19th century, and it probably refers to the fact that hats go out of fashion before they actually wear out