Green’s Dictionary of Slang

finished, ended, dead, no more.

1915		‘Ian Hay’ First Hundred Thousand (1918) 302: You say ‘Na pooh!’ when you push your plate away after dinner.
c.1916		‘Good-bye-ee’ 🎵 Bonsoir old thing, cheerio chin-chin, napoo, toodle-oo, good-bye-ee.
1916		B.E.F. Times 1 Dec. (2006) 132/1: Our Splendid New Serial. ‘Narpoo Rum.’.
1917		A.G. Empey Over the Top ‘Tommy’s Dict. of the Trenches’ 301: ‘Napoo-Fini.’ Tommy’s French for gone, through with, finished, disappeared.
1917		F. Dunham diary 21 July Long Carry (1970) 67: After the ‘strafe’ [we] returned to find some of our dugouts ‘Na pooh’.
1918		L.N. Smith Lingo of No Man’s Land 38: ESTAMINET [...] A place of higher grade than the ordinary saloon, where the fair Mademoiselle waits on Tommy, and where he is apt to sit and talk with his comrades until her ‘Nap peu, fini’ (no more; finished) is understood to mean closing time.
1921		Wodehouse Indiscretions of Archie Ch. iii: ‘I’ll let you have a suite and I’ll let you have your meals, but outside of that – nothing doing! Nothing doing! Do you understand what I mean?’ ‘Absolutely! You mean, “Napoo!”’.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 164: Napoo (or Napoo Finee): No more; all gone. Finished. Dead.
1929		J.B. Priestley Good Companions 525: Oh, poor old Liz! She was napoo before I got up to Newcastle.
1929		(con. WWI) R. Graves Goodbye to All That (1960) 155: Oh la, la, Johnny, napoo pozzy tomorrow!
1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.
1932		(con. WWI) L. Mann Flesh in Armour 91: ‘Where’s the bloody sigs?’ ‘Napoo [...] They were blown up’.
c.1943		‘Bill O. Lading’ You Chirped a Chinful!! n.p.: Napoo: No more.
1947		Eve. Teleg. (Dundee) 9 Sept. 4/1: French Basic Petrol Napoo. Abolition of the French private motorist’s petrol ration.
1951		Cusack & James Come in Spinner (1960) 262: Napoo. No tinkee findee.
2004		Guardian Rev. 3 July 3: Electronic discourse, billets-doux – vanished! Gone! Lost! Napoo!