Green’s Dictionary of Slang

£1 sterling.

1851		Chambers Edin. Jrnl XVI 192: ‘Eno dunop and a flah,’ says a new bidder. ‘A noc more,’ adds a competitor; and the lot is his for L.1, 15s.
1859		Hotten Dict. of Modern Sl. etc. 124: DUNOP, a pound.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].
1873		Sl. Dict. [as cit. 1859].
1936		Nottingham Eve. Post 9 Oct. 5/5: Back-slang [...] as in ‘dunop’ meaning a pound.
2001		J. Burkardt ‘Back Sl.’ Wordplay 🌐 doonups: pounds.