debauchery (n.)

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.