black tongue
chiefly US
noun
(Originally) any of various diseases or disorders exhibiting a dark discoloration of the tongue, especially blackleg in livestock or (more fully black tongue fever) typhus, typhoid, or meningococcal meningitis in humans; (in later use also) niacin deficiency, especially in livestock or domestic animals; pellagra.
Origin
Mid 19th century; earliest use found in Thomsonian Recorder.