Green’s Dictionary of Slang

(orig. US) goodbye.

1934		Weseen Dict. Amer. Sl. xiii. 173: [College slang] Abyssinia, I’ll be seeing you.
1939		R. Chandler ‘Pearls Are a Nuisance’ in Spanish Blood (1946) 117: ‘Check. Abyssinia.’ He hung up.
1950		C. Harris Three-Ha’Pence to the Angel 138: I must fly [...] Abyssinia!
1960		J. Mitford Hons and Rebels 159: You’ll find people generally say, ‘I’ll be seeing you’ instead of ‘goodbye’. [...] You may be able to raise a laugh by saying, ‘Abyssinia’.