A sausage, especially in British English; also something that bangs or makes noise, or informally, something excellent or impressive.
From 'bang' (to strike, make noise) plus the '-er' suffix. In British English, 'banger' refers to a sausage because it was prone to burst and 'bang' while cooking in early industrial processing.
The British term 'banger and mash' (sausage and mashed potatoes) comes from the tendency of poorly-made sausages to literally explode with a 'bang' in the pan—it's one of the few foods named after what happens to it while cooking.
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