From Longman Dictionary of Contemporary Englishold hatˌold ˈhat adjective [not before noun] OLD-FASHIONEDUSED TO/ACCUSTOMED TOif something is old hat, a lot of people have said or done the same thing before and it is therefore not new or interesting Most of this is probably old hat to you, isn’t it?
Examples from the Corpus
old hat• The movie's special effects now seem rather old hat. 