Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
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.).