a non sequitur
A statement that does not logically fit into the current conversation. Good luck getting answers from him—his every response to my questions so far has been a non sequitur. A: "I changed the oil in the car." B: "Well, that's a complete non sequitur, since I asked if you were hungry!"
a ˌnon ˈsequitur(from Latin, formal) a statement that does not seem to follow what has just been said in any natural or logical way: In the middle of a discussion about the weather, Liz started talking about fish. Everyone ignored the non sequitur completely.
The Latin phrase means ‘it does not follow’.