Green’s Dictionary of Slang

(US) nonsense, incomprehensible rubbish.

1863		letter q. in Wiley Life of Johnny Reb (1943) 207: Tell Bob he need not quit writing I can guess at his dutch if i cant read it.
1864, 1867, 1870		Hotten Sl. Dict.
1872		Schele De Vere Americanisms 83: English slang uses Dutch for any gibberish or unintelligible sounds.
1911		S. Ford Torchy 49: But there was Mr. Robert, waitin’ to talk Dutch to us.
1914		Bulletin (Sydney) 12 Dec. 17/1: We’ll just settle our breakfast an’ then you go along an’ talk Dutch to little Tommy Finch.
1963		H.E. Bates Oh! To be in England (1985) 338: It’s all Dutch to me.
1979		Maledicta III:2 158: Dutch, talk v phr Speak gibberish; from a play on ‘It’s all Greek to me’. [...] Dutch to me n phr Nonsense.
1989		H. Rawson Dict. of Invective (1991) 133: Dutch, to talk. To speak gibberish.