What does Boxing Day have to do with boxing?
"out of date," first recorded 1911. As a noun phrase, however, it had different sense previously. The "Classical Dictionary of the Vulgar Tongue" (1796) defines it as, "a woman's privities, because frequently felt."
Obsolete, old-fashioned: “Get with it, Murray; your methods are strictly old hat.”
Out of style; old-fashioned: Tubular stuff is now old hat (1911+ British)