Rotwelsch (n.)youdaoicibaDictYouDict[Rotwelsch 词源字典]
"jargon of thieves and vagabonds," 1841, from German Rotwelsch, literally "Red Welsh," from rot (see red (adj.1)) + Welsh because obscure and difficult. But the first element may be connected with Middle High German rot "beggar."[Rotwelsch etymology, Rotwelsch origin, 英语词源]