Green’s Dictionary of Slang

1. sixpence.

1861

(con. 1840s–50s) H. Mayhew London Labour and London Poor I 17/2: ‘Say a ‘exes’ (sixpence). – ‘Done!’.

2. the number six.

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 [...] for six exis.
1885		Edinburgh Eve. News 26 Dec. 3/5: ‘Eno’ is one; ‘owt,’ two; ‘eerth,’ three; ‘rouaf,’ four; ‘exif,’ five; and ‘xis’ pronounced exes, six.
1953		J. Franklyn Cockney 297: Exis (six).
1980		(con. 1930s) Barltrop & Wolveridge Muvver Tongue 35: ‘Yob’ for boy and ‘exis’ for six have been in general use throughout this century.

3. six pounds sterling.

1976		(ref. to 1930s–70s) R. Barnes Coronation Cups and Jam Jars 207: Xis – £6.
1989		J. Morton Lowspeak.

4. (UK Und.) a six-month prison sentence.

1989

J. Morton Lowspeak.