A plain suit refers to a basic outfit that is simple and unembellished. A few synonyms that you can use to describe it include "classic suit", "simple suit", "unadorned suit", "undecorated suit", "unembellished suit", and "unornamented suit". There are also colors to consider, such as a "black suit", "navy suit", "gray suit", "charcoal suit", "beige suit", and "brown suit". When it comes to style, you can opt for a "formal suit", "professional suit", "business suit", "conservative suit" or "traditional suit". These synonyms can help add variety and precision to your writing when describing a plain suit.