The term "under-thing" refers to an article of clothing that is worn underneath other garments. There are several different synonyms for this term depending on the specific garment being referred to. For example, underwear is the most commonly used term for undergarments worn beneath clothing. Other synonyms include undergarments, underclothes, lingerie, intimates, and foundation garments. These terms can refer to a range of clothing items, including bras, panties, undershirts, and more. It's important to find comfortable and supportive underthings that fit well, as they can impact the way your outerwear fits and feels.