Not canon; not official.
A theory and headcanon can be considered canon by an individual, however, not by the creator and therefore uncanon.

when you're hungover as shit after a spending a night drunk as a skunk

Another word for unbelievable.
It’s so bad that it’s unbelievable, uncanon

Adjective

1) To be lacking in Nick Canon like qualities.

2) Boring, not entertaining.

3) Failing at attempted humor - severely.