"Be offside", a term most commonly used in sports, refers to a player being in an illegal position during the game. There are several synonyms for the phrase such as "out of bounds", "off-base", "illegal", "unlawful", "unauthorized", and "prohibited". These words describe someone or something that is not following the rules or regulations. In the context of sports, a player is said to be "offside" or "out of bounds" when he or she breaches a rule, such as being in front of the ball when it is passed to them. These synonyms can be used interchangeably to describe a breach of rules or regulations in various contexts.