It's short for "north-north-west", ie. the direction between northwest and north. It's a reference to Hamlet's line, "I am but mad north-north-west. When the wind is southerly, I know a hawk from a handsaw." Paraphrasing Shakespeare, the meaning is that the person is insane only in certain situations. The reference to "north-north-west" is the direction of the wind.

It's a compass direction between due north and northwest -- seldom used in everyday life. In the passage, the idea is that it is a very specific direction. The speaker is suggesting that the man is mostly sane, but that he has one particular area in which he is insane.