I used to think this is irony, but after searching for the definition of "irony", it clearly isn't. It also isn't sarcasm. Here's an example:
Person 1: Hey, do you know Newton's second law?
Person 2: No, I'm a science student, not a law student.
Person 2 is making a joke by pretending that they do not know what Newton's second law is. Is there a name for this? Person 2 is being _____ ?
Such a person is answering in a deadpan manner:
deadpan, adj.: deliberately impassive or expressionless
Deliberately obtuse, maybe? Deadpan is probably better, but I just want to put it out there.
deliberately, adv.: with full awareness of what one is doing : in a way that is intended or planned
obtuse, adj.: stupid and slow to understand, or unwilling to try to understand:
If you put the two together, you get something along the lines of: Intentionally unwilling or slow to understand [for the purpose of being humorous or uncooperative]
(The example in the definition for obtuse (link 2) actually uses the construction 'deliberately obtuse.')
But I would still use deadpan because it's a) more common, and b) the humorous intent is inherent rather than implied.
It's a pun. It's more subtle than the usual sort, but it's a joke made by conflating two different meanings of the word "law" (a rule made by people, or a natural phenomenon). It feels less pun-like since in a normal pun, the meanings are completely different, whereas here they're similar.
A traditional pun is a groaner like "give peas a chance". But they can require more understanding, such as "politics and golf both depend on having a good lie". Or look at the things from 80 funniest puns that play a bit loose, like "Last night, I dreamed I was swimming in an ocean of orange soda. But it was just a Fanta sea".
