He could but smile.
Here, 'but' acts as-

• He could but smile.
- Here, 'but' acts as- খ) Adverb

• But (adverb)
- English meaning: only; no more than.
- Bangla meaning: শুধু; কেবল; মাত্র।

• অন্যদিকে,
• But (Noun): But me no buts.
• But (verb): But me no buts.
• But (Pronoun): Nobody but has his fault.

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