Green’s Dictionary of Slang

goodbye; thus as v. to say goodbye.

1908		‘O. Henry’ ‘The Clarion Call’ in Voice of the City (1915) 196: Tra-la!
1909		J. Ware Passing Eng. of the Victorian Era 249/1: Tra la la (Peoples’, 1880). Parting benediction – not too civil; possibly contemptuous. Died out about 1890. The phrase took its rise with a comic singer named Henri Clarke, whose speciality was imitating Parisians. Whether he invented ‘tra la la’, or heard it in Paris, or uttered by a Frenchman in London – he made a great hit with it as the burden of a chorus.
1917		O.O. McIntyre New York Day by Day 6 Aug. [synd. col.] ’Well, Mr Secretary, I hope you have nice weather for your war,’ and he tra-la-laed out.