unlined in the back
Refers to clothing, especially jackets or coats, that have no lining on the back part. Often used in tailoring or fashion contexts.
This jacket is unlined in the back, so you can wear it comfortably even in summer.
Compound of 背 (back) and 抜き (removal, omission), literally 'back omission'. The exact historical derivation is uncertain, but the meaning is transparent from its components.