A cigarette, especially a cheap or low-quality one, or slang for something that causes one to gasp with surprise or shock.
From gasp plus agent suffix -er (one who gasps). British slang, primarily from 20th century, likely referencing the harsh cough from cheap cigarettes.
Gasper as slang for cheap cigarettes reveals 1900s class consciousness in British English—expensive cigarettes were smoked quietly, but rough cheap ones made you gasp and cough audibly.
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