The word plain implies something simple or without decoration. Some synonyms that are associated with plain are unadorned, unembellished, or understated. Other synonyms that have a similar meaning to plain are straightforward, direct, blunt, or even bald. If plain refers to a person's appearance, the word can be replaced with words like homely, ordinary, or even unattractive, depending on the context. In some cases, the word plain can also suggest something dull or boring, and in this sense, it can be replaced with synonyms such as dull, mundane, or tedious. Whatever context the word plain is used in, there are plenty of synonyms that can be used to express the same idea.