noun
A kimono made of a single layer of fabric, worn in summer or as an undergarment. Contrasts with 袷 (あわせ), a lined kimono.
See also: 袷
夏は単物の着物が涼しくて快適だ。
In summer, an unlined kimono is cool and comfortable.
袷 is a lined kimono, while 単物 is unlined.
Compound of 単 (ひとえ, single layer) and 物 (もの, thing). The spelling 単衣物 explicitly adds 衣 (clothing).