Webster's Revised Unabridged Dictionary · 1913

Backset

Backset (bak"set`) , noun

[Back, adv. + set.]

1.
A check; a relapse; a discouragement; a setback.
2.
Whatever is thrown back in its course, as water.
Slackwater, or the backset caused by the overflow. — Harper's Mag

Backset , transitive verb

To plow again, in the fall; -- said of prairie land broken up in the spring. [Western United States]