1590s, "dig a new trench as a second line of defense," 1590s, probably a back-formation from retrenchment in the military sense. Related: Retrenched; retrenching.
"cut off, cut down, pare away" (expenses, etc.), 1620s, from obsolete French retrencher "to cut off, lessen, shorten" (Modern French retrancher, Old French retrenchier), from re- "back" (see re-) + Old French trenchier "to cut" (see trench). Related: Retrenched; retrenching.