A chain of logic with ends so remote that to anybody else it seems to be completely unrelated. Like a wiki walk, but through your head, and there's no history.
Example of a non sequitard:
Alice: Hmm... I think Grassy Knoll Dew would be better than Mountain Dew.
Bob: Would walruses fight for us in the zombie apocalypse if we gave them buckets?
Bob: Don't even try to figure out how I got there.