the.com/old hat

means Something so familiar, outdated, or overused that it no longer holds any interest or novelty.

from On record in English since the 1700s, with one vivid but unverified theory tracing it to the idea of an old, worn hat passed down and reshaped through many hands and heads — battered by overuse until it had nothing fresh left to offer. Another thread connects it to old slang senses where 'hat' carried a dismissive, well-worn connotation. The exact leap to 'stale and unoriginal' isn't pinned down with certainty, but the image of a hat gone shapeless from overuse fits the phrase perfectly.