Lewis & Short

in-dēfessus, a, um, adj., unwearied, indefatigable (poet. and post-Aug.): dextra, Verg. A. 11, 651: agendo, Ov. M. 9, 199: assiduus et indefessus, Tac. A. 16, 22: Germani ob prospera indefensi, id. ib. 1, 64.
Hence, indēfessē and indēfessim, advv., unweariedly (both post-class.).

        1. (α) Form indefesse: et benigne agere omnia, Aus. Grat. Act. 35; Spart. Hel. 5, 10.
        2. (β) Form indefessim: expetere bonorum amicitias, Sid. Ep. 3, 2; Claud. Mam. Stat. Anim. 1, 7.