σαθρός (sathros)

1. rotten, decayed, unsound, cracked , (Plato Philosophus)—;adverb, σαθρῶς ἱδρυμένος built on unsound foundations, (Aristotle Philosopher)
2. metaphorically, πρίν τι καὶ σαθρὸν ἐγγίνεσθαί σφι before any unsound thought comes into their heads, i. e. before they prove traitors, (Herdotus Historicus); ς. λόγοι (Euripides)

derivation uncertain (ML)