Green’s Dictionary of Slang

yoo-hoo! excl.

hello!

1941		K. Tennant Battlers 215: ‘Yo-ho!’ she yelled, full of excitement, waving her hat, running across the grass.
1960		Wodehouse Jeeves in the Offing 31: ‘Oh, hullo, Upjohn!’ I said. ‘Yoo-hoo!’.
1978		‘A.P. Gaskell’ All Part of the Game 152: ‘Yoo-hoo,’ auntie was standing on the porch and waving. ‘Yoo-hoo, Gordon.’.

In compounds

yoo-hoo (boy) (n.) [his camp shrieks of yoo-hoo! excl.]

(US) an effeminate homosexual.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1965		Guild Dict. Homosexual Terms 50: yoo-hoo (boy) (n., obs.): ‘A homosexual, as seen by the heterosexual; from his flamboyant greetings.’ (Slang.).
1972		B. Rodgers Queens’ Vernacular.