Lewis & Short

rĕ-tendo, di, tum, or sum, 3, v. a., to release from tension, to unbend, slacken, relax (very rare; syn.: relaxo, resolvo).

  1. I. Lit.: lentos arcus, to unbend, Ov. M. 2, 419; Stat. S. 4, 4, 30; in the part. perf.: arcus retentus, Ov. M. 3, 166: arcus retensus, Phaedr. 3, 14, 5.
  2. * II. Trop.: ea quoque, quae sensu et animā carent, velut alternā quiete retenduntur, i. e. unbend, relax (shortly before, remissio), Quint. 1, 3, 8.