Green’s Dictionary of Slang

sith-nom n.

a month.

1859		Hotten Dict. of Modern Sl. etc. 126: ERTH SITH-NOMS, three months, – a term of imprisonment unfortunately very familiar to the lower orders.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].
1873		Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.