The word under-coat means a layer of clothing, usually worn as a base layer to keep warm. There are various synonyms for the word under-coat, such as undershirt, undergarment, thermal, base layer, and long johns. These synonyms are often used to describe clothing that is worn close to the skin to provide an extra layer of warmth during colder temperatures. Under-coats can be made of different materials, such as wool, cotton, or synthetic fibers, depending on the intended use. These synonyms provide a variety of options for individuals to choose from when selecting the right under-coat to keep them warm.