The term "under coat" typically refers to a layer of clothing worn underneath an outer layer. Some synonyms for this term include "base layer," "undershirt," "long johns," and "thermal wear." These types of garments are designed to provide extra warmth and insulation during colder weather or outdoor activities. Other words that can be used to describe an under coat may include "insulating layer" or "base thermal layer." Regardless of the term used, the purpose of an under coat is to keep the body warm and protected from the elements, while also allowing for comfortable movement and flexibility.