Cannons or weapons positioned at the stern or rear of a ship, or the rear weapons in a military formation.
From 'after' (Old English 'æfter', meaning 'at the back') combined with 'guns' (possibly from Gunilda, a female name applied to cannons, or from gonne, an early cannon term). Naval military terminology.
Warships positioned their largest and most impressive guns as afterguns because they faced pursuing enemies—the most dangerous threat when trying to escape was always coming from behind.
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