Here's a breakdown:
* Argosy: This is an English word meaning a large merchant ship. It's not typically used in Welsh.
* Dim: This is Welsh for "no".
* Diolch: This is Welsh for "thank you".
If you are trying to express "no thank you" in Welsh, you would say:
Diolch yn fawr, ond na.
This translates to "Thank you very much, but no."
If you wanted to express "I don't need an argosy" in Welsh, it would be more complex and would likely require some context.