My dad always tells the same joke at dinner.

This joke cleverly plays with our expectations and a familiar phrase. When we hear "knee-slapper," we immediately picture a joke so hilarious it makes you physically react with a slap to the thigh. The irony kicks in when the punchline adds the crucial caveat: "if you've never heard it before." This instantly flips the meaning, suggesting the joke's humor is fleeting and entirely dependent on novelty, rather than inherent comedic genius.

The humor here is amplified by the universal experience of "dad jokes." Dads are famously (or infamously) known for their repertoire of predictable, often pun-based jokes that they trot out time and again, much to the exasperation (and sometimes secret amusement) of their families. The setup, "My dad always tells the same joke," immediately sets up this familiar scenario. The punchline then perfectly encapsulates the reality that while a joke might be genuinely funny on its first telling, its power diminishes with each subsequent repeat, especially for those who've heard it a hundred times. It's a comedic nod to the affectionate, yet eye-rolling, tolerance families often have for these beloved, if unoriginal, comedic stylings.