Green’s Dictionary of Slang

eno n.

one.

1859		Hotten Dict. of Modern Sl. etc.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 411/2: There’s people there talk backward – for one they say eno.
1870		Hotten Sl. Dict.
1882		Sydney Sl. Dict. (2 edn) 11: Eno - One.
1885		Edinburgh Eve. News 26 Dec. 3/5: ‘Eno’ is one; ‘owt,’ two; ‘eerth,’ three; ‘rouaf,’ four; ‘exif,’ five; and ‘xis’ pronounced exes, six.
2001		J. Burkardt ‘Back Sl.’ Wordplay 🌐 eno: one.