pre-emptive-senility
Explanation
The Joke
A father tells his son that senility runs in their family and that it is probably only another decade or so before the son starts seeing symptoms. But rather than offering comfort or advice, the father announces that he plans to make the problem "substantially worse" by spending his retirement deliberately changing the meaning of frequently-used words in his own vocabulary.
He demonstrates this by explaining that he has already spent the last three "sporks" (years? months?) changing one word per "pants" (sentence? paragraph?). He says that "pants after pants" his speech will become more undecipherable. He then asks, "Do you banana what I am saying?" -- replacing "understand" with "banana." The son turns to someone else and asks, "Is it senility that runs in our family, or dickishness?" The father replies, "That is none of my porosity, stickleback" -- now fully committed to his scheme of word substitution.
The Humor
The comedy comes from the father weaponizing the expectation of senility as cover for deliberate mischief. Instead of dreading cognitive decline, he is proactively creating his own version of it for fun, which is both absurd and oddly logical -- if people are going to assume you are senile anyway, you might as well enjoy the process. The escalating word substitutions (sporks, pants, banana, porosity, stickleback) are funny because they are clearly intentional yet mimic the kind of confused speech associated with actual cognitive decline.
The son asking whether the family trait is "senility or dickishness" is the perfect punchline because it reframes the entire premise. The father's final response, entirely in his invented nonsense vocabulary, confirms that it is very much the latter -- and that he is thoroughly enjoying himself.