General opposites:
* Apathy: Lack of interest, enthusiasm, or concern.
* Indifference: A lack of feeling or concern.
* Laziness: Disinclination to work or exert oneself.
* Lethargy: A state of sluggishness or inactivity.
More specific opposites:
* Disdain: Contempt or scorn. (Opposite of zealousness in pursuit of something)
* Reluctance: Unwillingness or hesitation. (Opposite of zealous commitment)
* Hesitation: The act of pausing or delaying. (Opposite of zealous action)
* Discouragement: A loss of hope or confidence. (Opposite of zealous spirit)
The best opposite word for "zeal" will depend on the specific situation and the nuance you want to convey.