My mother-in-law said she'd dance on my grave.

This joke gets its chuckle from a clever twist of expectation, playing on both an idiom and a literal interpretation. The setup presents a classic comedic scenario: the often-antagonistic relationship with a mother-in-law, expressed through the common, albeit dark, idiom of "dancing on one's grave." This phrase is universally understood to mean celebrating someone's death with malicious glee. The humor mechanism kicks in when the punchline takes this metaphorical threat and counters it with a very literal, practical solution, effectively pulling the rug out from under the mother-in-law's spiteful plan.

The "mother-in-law joke" is a time-honored comedic staple, deeply rooted in cultural stereotypes that portray this family member as meddling or critical, a tradition that dates back centuries and appears in folklore and literature across various societies. While often exaggerated for comedic effect, it taps into relatable domestic tensions. The idea of "dancing on someone's grave" is also a well-established expression of extreme animosity. The punchline's mention of burial at sea grounds the joke in a real, though less common, funeral practice, adding an element of unexpected practicality to the absurd situation and making the thwarted revenge even funnier.