"Below board" is a term that serves as the antonym for "above board." It implies that something is dishonest, unethical, or illegal. It describes a situation where a person is hiding something from others or being secretive about their actions. "Below board" means going against the rules and regulations or breaking the law.
Another term that can be used as an antonym for "above board" is "sneaky." This word implies that someone is being deceitful on purpose, often with the intention to benefit themselves. A person who is acting in a sneaky manner may be trying to hide something or being manipulative for their gain.
In contrast to above board, synonyms include honest, transparent, and straightforward, while antonyms include sneaky and below board.