Green’s Dictionary of Slang

goodbye (cf. chow phr.).

[1919		E. Hemingway letter 30 Apr. in Baker Sel. Letters (1981) 30 24: Ciaou Kid].
1964		J. Blake letter 30 June in Joint (1972) 231: Ciao, Charlie.
1974		(con. late 1940s) E. Thompson Tattoo (1977) 3556: ‘Ciao!’ she trilled after him.
1983		T. Paulin ‘Martello’ in Liberty Tree 51: ...must hatch a snifter: Ciao now, old cock.
1999		K. Sampson Powder 18: Better dash. Do say you’ll come tomorrow. Ciao, honey.