Green’s Dictionary of Slang

1. a sexual offender.

1945		O.E. Burton In Prison 146: The thieves [...] were quite emphatic that there was nothing else to be done [except imprisonment] with the ‘sexos’ and murderers. The ‘sexos’ thought that while they were a special and rather superior class needing quite different treatment, common thieves deserved what was coming to them .
1963		R.H. Morrieson Scarecrow 113: ‘Maybe it’s only me,’ said Prudence. ‘Maybe he’s only laying for me. Maybe he’s a sex-oh.’ [DNZE].
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxix 4/5: sexos or seckos: Prisoners sentence for sex crimes against young persons. The most despised of all prisoners.
1998		Guardian 5 Sept. 35/2: Specific to New Zealand (you don’t want to be a sexo or kid fucker in parrie Max, unless you’re cunning as a shithouse rat.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 162/2: sexo n. a sex offender.

2. someone seen as over-sexed or obsessed with sex.

1951		joke in a Victoria University College Extravaganza. ‘Dial 60 60 60’ [i.e. /’seks.AU/] [DNZE].
1965		L. Dunne Goodbye to The Hill (1966) 130: A verbal sexo, that’s what Miss O’Boyle was.
1976		F. Sargeson Sunset Village 49: There was a young hound just the other day, and another of the cold-ice breed, a sexo who had talked back, quoted somebody...who had laid it down, twice a week–to keep a man healthy [DNZE].
1988		McGill Dict. of Kiwi Sl. 98/1: sexo used to suggest another male is well-sexed, randy, ready to go sexually, often affectionately or as a greeting.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

3. attrib. use of sense 2.

1953		I. Hamilton Till Human Voices Wake Us 39: The interminable boob stories [...] Well, Lofty would say after a pause in the talk, time for the sexo hour.
1959		G. Slatter Gun in My Hand 179: Sefto you old sexo! How the hell are you? [Ibid.] 230: Amy won’t open the window because you’re too sexo she says.