What is called for a metal piece that goes under TONGUE?

There isn't a specific, common term for a metal piece that goes under the tongue. The term you are looking for might depend on the specific function or purpose of this piece:

* Dental appliance: If it's part of a dental appliance, it might be called a lingual bar, lingual plate, or sublingual retainer.

* Jewelry: If it's a piece of jewelry, it might be called a tongue ring, tongue stud, or tongue barbell.

* Medical device: If it's a medical device, it could have various names depending on its function, such as a lingual stimulator or sublingual implant.

To give you a more accurate answer, I need more context about what this metal piece is for.

Could you please provide more details about its purpose or function?