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Old English rustig; see rust (n.) + -y (2). Cognate with Frisian roastich, Middle Dutch roestich, Dutch roestig, Old High German rostag, German rostig. "In the 16th and 17th centuries frequently used as a term of general disparagement" [OED]. Of bodily skills, "impaired by neglect," from c.1500; of mental qualities, accomplishments, etc., first attested 1796.