If "keen" means enthusiastic or eager:
* Apathetic: Lacking interest or enthusiasm.
* Indifferent: Not caring one way or the other.
* Uninterested: Not showing any interest.
* Reluctant: Hesitant or unwilling.
* Disinclined: Not inclined to do something.
If "keen" means sharp or sharp-witted:
* Blunt: Not sharp or pointed.
* Dull: Not sharp or intelligent.
* Obtuse: Slow to understand or learn.
* Unperceptive: Lacking in understanding or insight.
If "keen" means intense or strong:
* Weak: Lacking in strength or intensity.
* Mild: Not intense or strong.
* Dim: Not bright or intense.
To provide the most accurate opposite, please tell me the context in which you are using the word "keen".