1590s, "to entice, seduce, lead astray" (from allegiance, family, etc.), from French débaucher "entice from work or duty," from Old French desbaucher "to lead astray," a word of uncertain origin.
Supposedly it is literally "to trim (wood) to make a beam" (from bauch "beam," from Frankish balk or some other Germanic source akin to English balk (n.)). The notion of "shaving" something away, perhaps, but the root is also said to be a word meaning "workshop," which gets toward the notion of "to lure someone off the job;" either way the sense evolution is unclear.
The more specific meaning "seduce from virtue or morality, corrupt the morals or principles of" is from c. 1600, especially "to corrupt with lewdness, seduce sexually," usually in reference to women. Intransitive sense "indulge in excess in sensual enjoyment" is from 1640s. As a noun, "a bout of excessive sensual pleasure," c. 1600.
"seduced or corrupted from duty or virtue, vitiated in morals or purity of character," 1590s, past-participle adjective from debauch (v.). Related: Debauchedness.
"habitually lewd or profligate person, one addicted to vicious indulgence in sensual pleasures," 1660s, from French débauché "debauched (person)," noun use of past participle of debaucher (see debauch).
Debauchee, n. One who has so earnestly pursued pleasure that he has had the misfortune to overtake it. [Ambrose Bierce, "Devil's Dictionary," 1911]
large salamander of the Ohio and Mississippi rivers, 1812, supposedly so called for its ugliness. The meaning "reckless debauch, drunken frolic" is from 1889.
c. 1500, "act of adulterating; state of being debased by mixture with something else," generally of inferior quality, from Latin adulterationem (nominative adulteratio) "an adulteration, sophistication," noun of action from past-participle stem of adulterare "corrupt, falsify; debauch; commit adultery," from ad "to" (see ad-) + alterare "to alter" (see alter), though Watkins explains it as ad alterum "(approaching) another (unlawfully)." Meaning "a result of adulterating" is from 1650s.
Old English hær "hair, a hair," from Proto-Germanic *hēran (source also of Old Saxon, Old Norse, Old High German har, Old Frisian her, Dutch and German haar "hair"), perhaps from PIE *ghers- "to stand out, to bristle, rise to a point" (source also of Lithuanian šerys "bristle;" see horror).
Spelling influenced by Old Norse har and Old English haire "haircloth," from Old French haire, from Frankish *harja or some other Germanic source (see above). Hair-dye is from 1803. To let one's hair down "become familiar" is first recorded 1850. Homeopathic phrase hair of the dog (that bit you), remedy from the same thing that caused the malady, especially a drink on the morning after a debauch, 1540s in English, is in Pliny.