In medical contexts, a thick or viscous bodily fluid; historically used in humoral medicine to refer to thick blood or other body humors.
From Latin 'crassus' (thick), used in medieval and Renaissance medicine to describe the consistency of bodily fluids. It's a direct borrowing of the Latin adjective used as a noun.
Medieval doctors blamed thick blood ('crassis') for all sorts of illnesses and would actually bleed patients to thin it out—thankfully our medicine got way better!
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