"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.
Entries linking to sebum
soap n.
Middle English sope, from Old English sape "soap, salve," anciently a reddish hair dye used by Germanic warriors to give a frightening appearance, from Proto-Germanic *saipon "dripping thing, resin" (source also of Middle Low German sepe, West Frisian sjippe, Dutch zeep, Old High German seiffa, German seife "soap," Old High German seifar "foam," Old English sipian "to drip"), from PIE *soi-bon-, from root *seib- "to pour out, drip, trickle" (perhaps also the source also of Latin sebum "tallow, suet, grease").
Romans and Greeks used oil to cleanse the skin; the Romance words for "soap" (Italian sapone, French savon, Spanish jabon) are from Late Latin sapo "pomade for coloring the hair" (first mentioned in Pliny), which is a Germanic loan-word, as is Finnish saippua. The figurative meaning "flattery" is recorded from 1853.
sebaceous adj.
1728, "secreting sebum;" 1783, "pertaining to tallow or fat;" from Latin sebaceus "of tallow," from sebum "tallow, grease" (see sebum). Meaning "oily, greasy, fatty" is from 1783.