noun, adjective - One's birthday suit is one's bare skin or the state of being nude or without clothes, attire or swimwear of any kind.
One's bare skin comes from the fact that a person does not have any clothes on the minute they are born.
He did his birthday swim in his birthday suit.
Sydney Skinny Ocean Swim
The Sydney Skinny is the world's first nude ocean swim held at Cobblers Beach, Middle Head Sydney Harbour National Park in Australia, founded by Nigel Marsh and organized by Kate Hughes of Event Aid Australia. Taryn Brumfitt is the event ambassador for 2014. Over 750 people swam on 17 February 2013 for the inaugural Sydney Skinny under a sparkling summer day and a fabulous crowd who were eager and excited to get down to Cobblers Beach and enjoy the fun. The conditions in the water were calm and many swimmers have said The Sydney Skinny was their first ocean swim and that they were so thrilled to have completed the course.