What is the name of the Zoroastrian sacred undergarment?

The Zoroastrian sacred undergarment is called the Sudreh.

Also spelled Sedreh, Sudre, or Sudra, the Sudreh is a usually white ritual shirt worn by initiated Zoroastrians. It is worn together with the Kushti, a sacred cord tied around the waist. The pair forms an important part of Zoroastrian religious dress and is traditionally worn beneath ordinary clothing.

The Sudreh has a small front pocket that is traditionally understood as symbolically collecting a person's good deeds. Its purpose is not merely practical clothing: Zoroastrian tradition treats it as a spiritual reminder and a protective garment associated with resisting evil. Children commonly receive the Sudreh and Kushti during the Navjote initiation ceremony in India, or the comparable Sudreh-Pushi ceremony among Iranian Zoroastrians.

A common mix-up is confusing the Sudreh with the Kushti. The Sudreh is the shirt; the Kushti is the cord. The Dhoti is a South Asian garment, while a Kippah and Tallit are Jewish religious items, not Zoroastrian clothing.