It's possible you've heard someone use the word "butty" in a Welsh context, but it's likely they were using the standard English term for a sandwich. There's no specific Welsh slang equivalent for "butty."
However, the Welsh language does have its own words for sandwich:
* Sgwrs: This is a more formal word for sandwich.
* Sgwrs bach: This means "small sandwich".
* Bara wedi'i fwyta: This literally translates to "bread that has been eaten" and can be used to refer to a sandwich.
If you are looking for a word for sandwich in Welsh slang, you could use "sgwrs bach" or "bara wedi'i fwyta", but these aren't particularly common in everyday speech. It's more likely that someone would simply use the English word "sandwich" or "butty" in Welsh conversation.