Green’s Dictionary of Slang

1. the penis, esp. when small; also attrib.

1929		Blind Lemon Jefferson ‘Yo Yo Blues’ 🎵 Go yo-yo with some other man.
1930		Blind Willie McTell ‘Talking to Myself’ 🎵 Make me break my yo-yo string / Honey I ain’t going to be / your old work ox no more.
1963		L.F. Freed Crime in S. Afr. 83: Bawdy songs like ‘Johnny had a yo-yo.’ .
1967		L. Bruce Essential Lenny Bruce 119: Mr. Yo-Yo can’t get his yo-yo up.

2. (US) an unpredictable or inconsistent person whose moods and actions go up and down, thus a fool; also as adj., foolish.

1947		H. Boyle 19 May [synd. col.] One of the yo-yo boys sitting next to me [W&F].
1959		E. De Roo Go, Man, Go! 102: Those yo-yos are kidding me.
1959		H. Ellison ‘Sally in Our Alley’ in Gentleman Junkie (1961) 117: They took me down in a prowl car, sitting between two yo-yo cops.
1960		G. Swarthout Where the Boys Are 191: If you think I’ll screw up the last two days of my vacation for a stunt as Boy Scout and yo-yo as this, you are stark raving.
1968		K. Brasselle Cannibals 271: One night with that yo-yo would drive her to the funny farm.
1978		T. O’Brien Going After Cacciato (1980) 28: You asshole, he thought. You ridiculous little yo-yo.
1989		T. Blacker Fixx 184: Chinless yo-yos in their cups.
1996		S. Frank Get Shorty [film script] The Colombians are in L.A. Seems they all upset about their money. That ain’t enough, as a bonus, it turns out the yoyo was Escobar’s nephew.
2012		K. Shea ‘Pride’ in C. Rhatigan and N. Bird (eds) Pulp Ink 2 [ebook] It’s that stuff earth-crunchy yo-yos used to make [...] — Bio fuel?
2020		A. Trebek The Answer Is 40: [T]hese yo-yos who are just throwing their weight around for whatever reason.

3. (US) in sports, a sequence of inconsistent, back-and-forth results.

1969

L. Schecter Jocks 224: [T]he boxing crowd was almost unanimous in picking the first two Patterson-Johansson yo-yos the wrong way.

4. (US campus) a bisexual person.

1986

Eble Campus Sl. Nov. 9: yoyo – bisexual: That yoyo dropped her boyfriend for her new girlfriend.