Dictionary.net

• The grayish crust or buff, varying in thickness, observed on blood drawn from a vein during the existence of violent inflammation, pregnancy, &c. It is particularly manifest in pleurisy, and hence one of its names. For its production, it appears to be requisite, that there should be an increase in the proportion of the fibrinous element of the blood over that of the red corpuscles, with increased aggregation of those corpuscles. Under such circumstances, the buffy coat assumes a concave appearance on its upper surface, and the blood is, therefore, said to be cupped.

1846 - Medical lexicon: a dictionary of medical science
By Robley Dunglison