rolling
Explanation
The Joke
An older man (a father figure) angrily tells a younger man that "Your grandfather would be rolling in his grave!" -- a common idiom meaning a deceased person would be deeply upset or outraged by something happening in the present. The younger man looks uncomfortable. The caption below then reveals: "Dad expressed enthusiasm for my automatic coffin tumbler." The son has invented a literal device that makes coffins rotate, and his father's exclamation -- which sounds like furious disapproval -- is actually a positive product review.
The entire joke hinges on the double meaning of "rolling in his grave." In everyday usage, it expresses moral outrage on behalf of the dead. But the son has literalized the metaphor by building a machine that physically rotates coffins, so the father is genuinely excited that grandpa's coffin will be spinning as advertised.
The Humor
This is a clean example of a literalized-idiom joke, a staple of SMBC's style. The comic is structured as a misdirect: the first panel presents what appears to be a scene of familial disappointment, with the father's angry face and the son's worried expression reinforcing that reading. The punchline in the caption completely reframes the scene. The absurdity of an "automatic coffin tumbler" as a real product -- and the idea that a father would be enthusiastic about it -- adds an extra layer of comedy beyond the wordplay itself.