double Dutch; Dutch noun unintelligible speech UK,1876 Double Dutch is also a secret language in which words are disguised from understanding.
Although a lot of this was Dutch to me at the time, my friend knew how to “tell the fanny” (tell the tale)[.] — Butch Reynolds, BrokenHearted Clown, p. 31, 1953
If it’s all Greek or double Dutch, Bazza’s the man. — TheGuardian, 16 August 2004