water bomb1
/ˈwɔːtə ˌbɒm/noun
1A balloon, paper bag, etc., that is filled, or designed to be filled, with water, and is thrown or dropped so that it bursts and disperses its contents on impact, usually as part of a water fight or as a prank.
2A large balloon or bag filled with water and dropped from an aircraft as a firefighting measure, especially against forest or bush fires.
3A depth charge. Now historical.
Origin
Mid 18th century (in an earlier sense). From water + bomb.
water-bomb2
/ˈwɔːtəbɒm/verb
[with object] To attack or strike with a water bomb, or (more generally) a large quantity of water; to direct a water bomb at; especially to release a large quantity of water on to (a fire) from the air as a firefighting measure.
Origin
1940s; earliest use found in The Hammond Times. From water + bomb, after water bomb.