An antonym of 'Zealous' is -

The antonym of 'zealous' is- Apathetic.

• Zealous (adjective)
- English Meaning: Feeling or showing strong and energetic support for a person, cause, etc.
- Bangla Meaning: উদ্দীপনাময়; গভীর অনুভূতিবহ; আগ্রহোদ্দীপক।

• Synonyms: Earnest (অত্যন্ত; উৎসুক; আন্তরিক), Enthusiastic (অত্যুৎসাহী), Ardent (অতিশয় আকুল বা অত্যন্ত উৎসাহী).

• Antonyms: Apathetic (সমবেদনা বা আগ্রহহীন; উদাসীন), Passionless (উদাসীন; আবেগহীন), Indifferent (নিঃস্পৃহ; উদাসীন).

• Other forms:
- zealously (adverb)
- zealousness (noun)

• Example Sentence:
1. He is a zealous supporter of the government's policies.
2. He thought the reporters were too zealous in their attempts to get into the event.

Source:
1. Accessible Dictionary by Bangla Academy.
2. Cambridge Dictionary.
3. Merriam-Webster Dictionary.