General synonyms:
* Careless
* Indifferent
* Listless
* Sluggish
* Lethargic
* Unenthusiastic
* Half-hearted
* Apathetic
More specific synonyms:
* Flippant: This implies a careless disregard for seriousness or importance.
* Nonchalant: This suggests a casual indifference, often to the point of being cool or unconcerned.
* Remiss: This suggests a failure to be diligent or attentive to one's duties.
* Lax: This implies a lack of strictness or firmness, often in a way that suggests leniency.
* Negligent: This suggests a failure to take proper care or attention to something.
The best synonym for "lackadaisical" will depend on the specific context and meaning you want to convey.