Definition of sack-back in English:

sack-back

(also sacque-back, sac-back)

Pronunciation /ˈsakbak/

noun

Fashion
  • 1On a garment, especially a woman's gown: a loose-fitting back, with panels of fabric falling, typically in two box pleats, from just below the shoulders to the hem.

    The style was popular in the 18th century; since then usage of the word has been historical.

  • 2On a coat, jacket, dress, etc.: a loose back falling from shoulder to hem without a defined waistline, evoking or inspired by the 18th-cent. style.

adjective

Fashion
  • 1Of a coat, jacket, dress, etc.: having a loose back falling from shoulder to hem without a defined waistline; of or characteristic of such a garment.

  • 2historical Of a garment, especially a woman's gown: having a loose-fitting back with panels of fabric falling, typically in two box pleats, from just below the shoulders to the hem; of or characteristic of such a garment.

Origin

Late 18th century; earliest use found in The Public Advertiser. From sack + back.