Green’s Dictionary of Slang

a form of termination, e.g. death, disaster.

1917		Weston & Lee [perf. Florrie Forde] Good-bye-ee! 🎵 Bonsoir, old thing, cheerio chin chin, / Nah-poo, toodle-oo, good-bye-ee.
1926		(con. WWI) ‘Sapper’ Shorty Bill 72: Shells, bombs, bullets – they take others and pass you by. But sooner or later, it will be ‘nah-poo.’.
1928		(con. WWI) H.F. Cruikshank ‘So This Is Flanders!’ Battle Stories July 🌐 If this guy was goina giggle out loud on patrol, we figured we was settin’ pretty for a na-poo any ol’ time.