Fromback +‎set.

backset (pluralbacksets)

1. Acheck; arelapse; adiscouragement; asetback.
2. Whatever is thrown back in its course, such aswater.
   • (Can wedate this quote?),Harper's Magazine:

     Slackwater, or thebackset caused by the overflow.

backset (third-person singular simple presentbacksets,present participlebacksetting,simple past and past participlebackset)

1. (US, especiallyWestern US) Toplow again in thefall; said ofprairie land broken up in thespring.

• “backset”, inWebster’s Revised Unabridged Dictionary, Springfield, Mass.:G. & C. Merriam,1913,→OCLC.

• backest,backets,set back,setback