You probably know the French question/answer pair Ça va? – Ça va, translated by “How are you?” “Good”. Ça baigne has the exact same meaning, even though baigner means “to bathe”. The origin of the expression, that dates backs to the 1950s, is not very clear. It might have been associated with the beach, where people would go bathing but also are often baignés (bathed) in suntan oil!
French with Valerie 