Skinny-dipping is swimming with no clothes on. People do it in a private pool, a quiet lake, or the sea when they think nobody is watching. It is informal and a little playful — the sort of word friends use, not a legal or medical word. It can sound like fun if everyone agreed; it is not fun, and not harmless, if someone did not. The skinny part is old informal English for “naked.” It does not mean the person is thin. You can also say go skinny-dipping or skinny-dip.

裸泳（非正式、帶點開玩笑。這裡的 skinny 是「光著身子」，不是「瘦」）