What is the prefix for buttoned?

There isn't a single prefix specifically for "buttoned". However, depending on the context, you could use several options:

* Button-: This is the most straightforward prefix, indicating something related to buttons. Example: button-down shirt.

* Buttoned-: This is a straightforward way to indicate that something is in the state of being buttoned. Example: buttoned-up coat.

* Fastened-: This is a more general prefix that can be used for anything that is closed or secured, including buttoned items. Example: fastened door.

The best option will depend on the specific context and the desired meaning.