Green’s Dictionary of Slang

non-committal, inconclusive, dull.

1963		Princeton Alumni Wkly 64 195: To many of you this news may seem a bit ho hum.
1969		Daily Colonist (Victoria, B.C.) 27 Sept. 2/3: People are pretty ho-hum on most parts of the [Vancouver] Island right now but if anything does happen, they will be the first to cry for damages [OED].
1974		N.Y. Mag. 30 Dec.-6 Jan. 11: Emmanuelle— Ho-hum soft-core French porn, based on a 1957 novel (supressed by DeGaulle's regime, but since surfaced).
1988		Scouting (US) Sept. 14: A lot of people, though, were pretty ho-hum about it all.
1990		J. Wambaugh Golden Orange (1991) 153: ‘What is this?’ she said, finally. ‘An H.F.H. good-bye?’ [...] ‘Ho-fucking-hum. Is that all I’m going to get out of you, old son?’.
2005		J. Stahl I, Fatty 77: It didn’t take long around Mack for the rankest behavior to seem ho-hum.
2018		(con. 1963) L. Berney November Road 30: [T]his [i.e. the JFK assassination] wasn’t just another ho-hum murder, some shoe-leather wiseguy popped in a back alley.
2023		(con. 1962) J. Ellroy Enchanters 47: The job vibed [...] ho-hum revelation.