wearing kimono without hakama (for men); dressing casually in kimono
Refers to a man wearing a kimono without the formal hakama trousers, giving a relaxed, informal look. Can also imply casual dress in general, but the kimono context is primary.
夏祭りには着流しで出かける男性も多い。
Many men go to summer festivals in kimono without hakama.
彼はいつも着流しで、堅苦しい服装を好まない。
He always dresses casually in kimono and dislikes stiff formal wear.
The noun is derived from 着流す, “to wear a kimono casually without hakama,” with 流す contributing the sense of leaving the dress informal or unarranged.