Green’s Dictionary of Slang

a warning cry on sighting a policeman, meaning ‘be off!’; ext. just to mean ‘go away!’.

1856		H. Mayhew Great World of London I 6: Curly! Nommus! (be off).
1859		Hotten Dict. of Modern Sl. etc. 66: NAMUS, or namous, some one, i.e. ‘be off, somebody is coming.’.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 17/1: Even if a stranger should advance, the cry is given of ‘Namous’. [Ibid.] I 24/1: Nommus ... Be off.
1876		C. Hindley Life and Adventures of a Cheap Jack 108: Aaron suddenly exclaimed ‘namus,’ thereby meaning, cut, run, take care of yourselves.
1894		G.F. Northall Folk-Phrases of Four Counties 40: Nammus! = Be off.
1977		D. Powis Signs of Crime 195: Nommus! Costermongers’ and pitch and toss players’ warning cry upon the approach of police (London backslang).