Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
indēfessē and indēfessim, advv., v. indefessus fin.
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.).