Resistant to the effects of arsenic or able to withstand arsenic treatment without harm.
From arsenic + fast (meaning firmly fixed or resistant). This is a technical term from agriculture and toxicology, combining Old English 'fæst' meaning secure or immovable.
Some insect populations evolved 'arsenfast' resistance through repeated exposure to lead arsenate sprays—one of the first documented examples of pesticide resistance, showing that organisms adapt faster than we expect.
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