The phrase "birthday suit" is a casual or humorous term for being naked. While it is a common expression, it may not always be appropriate for certain situations. There are several other synonyms that can be used in place of "birthday suit" which include; being in the buff, in the altogether, in the nude, in the raw, in the skin, in the altogether, and stark naked. These expressions are often used in a playful or humorous context, and it is important to consider the setting before using them. It is always advisable to use appropriate language when communicating with others to avoid offending anyone or creating an uncomfortable atmosphere.