It refers to the washing of oneself in icy water, the river, sea or waterfall in Shinto or Buddhism when one's body is polluted, or before or during an important Shinto ceremony.
神道や仏教で自分自身の身に穢れのある時や重大な神事などに従う前、又は最中に、自分自身の身を氷水、滝、川や海で洗い清めること。