Tiny flakes of dead skin that naturally shed from animals (especially pets) and humans; or anger/annoyance.
From Old Norse 'danðr' (scurf/flaking). Originally Scandinavian, entered English around the 1600s. The 'anger' meaning comes from the phrase 'get one's dander up' from unknown origins.
Pet dander is a major allergen, but it's not actually from fur — it's the dead skin cells and dried saliva on the fur, which is why there's no truly 'hairless' hypoallergenic dog, just dogs that shed less dander.
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