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There are four forms — more, actually — and meanings for this verb in French (I rank them below according to the number of occurences in daily French, as indicated by a very complete dictionary I have):
1. (se) baigner (reflexive verb): bathe, swim, to go swimming - for example: « je vais me baigner avec des copains (or: quelques copains à moi) », which means «I'm going swimming with some of my buddies»;
2A. baigner (transitive): wash somebody, bathe, give somebody a bath - for example: « il vaut mieux baigner son bébé avant de le nourrir », which means «it's better to bathe your baby before a feeding»;
2B. baigner (transitive): wash, bathe - for example: « Comment prendre un bain au Japon selon les règles en usage », which means «How to take a bath in Japan according to etiquette rules»;
3. baigner (intransitive): be immersed or immerse oneself - for example: « baigner dans la graisse », which means «to be swimming in grease».
+ There is also the idiomatic answer to «How are you doing?» (Comment ça va ?): « — Ça baigne ! » (I'm doing fine.), that I have often heard in Switzerland and France.