Green’s Dictionary of Slang

1. (milit.) to kill.

1915		‘Ian Hay’ First Hundred Thousand (1918) 302: Poor Bill got na-poohed by a rifle-grenade yesterday.
1918		Aussie (France) IX Dec. 6/1: ’E surrounded about seven Fritzes on ’is own, an’ napood the lot. I finished seventeen myself that night. But six cowardly, blanky Alleymans rushed old Tommo.
1921		N&Q 12 Ser. IX 425: Napoo’d. Done for; put out of action; killed.

2. to die.

1918

Aussie (France) IX Dec. 6/1: ’N what do he say before he – what you call – napoo?