I saw this in a Dilbert strip.

The word is ennui, which is pronounced “ahn-wee”, and it means a feeling of utter weariness and discontent resulting from satiety or lack of interest; boredom.

The word satiety means the quality or state of being fed or gratified to or beyond capacity.

In other words, when you are stuck in an endeavor that is no longer challenging, when the path you are on has no more to offer you, ennui is the inevitable result.