What is "rotwelsch"

"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 or Gaunersprache (, "criminal language") is a secret language, a cant or thieves' argot, spoken by covert groups primarily in southern Germany and Switzerland.