summer overcoat; light jacket
Rare term for a light coat or jacket worn in summer. In modern Japanese, 夏コート or 薄手のジャケット are more common.
In an old novel, the expression 'to put on a summer overcoat' appears.
Compound of 夏 (summer) and 外套 (overcoat). The exact historical derivation is uncertain, but it follows a standard pattern of seasonal clothing terms.