One of the mysteries of the English language finally explained.
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.
Mid 18th century (in an earlier sense). From water + bomb.
water bomb/ˈwɔːtə ˌbɒm/
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