To say of something that it is old hat is to say that it is old, or well known. Wondering about the origin of this curious phrase, I turned to Robert Hendrickson, Encyclopedia of Word and Phrase Origins, 2nd ed. (2004), p. 529. What I found there surprised me:
Today old hat means out of date or not new, and it has meant this for at least a century. But back as early as 1754 it was "used by the vulgar in no very honorable sense," as Fielding put it. It then meant, in Grose's punning definition from his 1785 Classical Dictionary of the
Vulgar Tongue: "a woman's privities: because frequently felt."
This is no doubt interesting, but how does it explain the origin of the the adjectival phrase 'old hat'? That 'old hat' was once used as a noun by a certain class of people to refer to "a woman's privities" does nothing to show the origin of 'old hat' as currently used.