seborrhea (n.)

"a secretion of the sebaceous glands," 1728, from medical use of Latin sebum "sebum, suet, grease," which is perhaps related to sapo "soap" (see soap (n.)), but de Vaan is skeptical and gives it no etymology.

Proto-Indo-European root meaning "to flow."

It forms all or part of: amenorrhea; catarrh; diarrhea; gonorrhea; hemorrhoids; maelstrom; rheo-; rheology; rheostat; rheum; rheumatic; rheumatism; rheumatoid; rhinorrhea; rhythm; seborrhea; stream.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sravati "flows," srotah "stream;" Avestan thraotah- "stream, river," Old Persian rauta "river;" Greek rhein "to flow," rheos "a flowing, stream," rhythmos "rhythm," rhytos "fluid, liquid;" Old Irish sruaim, Irish sruth "stream, river;" Welsh ffrwd "stream;" Old Norse straumr, Old English stream; Lettish strauma "stream, river;" Lithuanian sravėti "to trickle, ooze;" Old Church Slavonic struja "river," o-strovu "island," literally "that which is surrounded by a river;" Polish strumień "brook."