The word "不雅" is formed by directly combining the character "不", meaning "not", as a negating prefix with the character "雅", meaning "elegant" or "proper". This pairing creates a compound where the negation modifies the root meaning to produce the sense of "not elegant" or "not proper", specifically used to denote indecency or obscenity. The formation is a straightforward lexical construction where the prefix inverts the adjective's meaning.