E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Avernus (Greek, a-ornis, without a bird).
A lake in Campania, so called from the belief that its sulphurous and mephitic vapours killed any bird that happened to inhale them. Poets call it the entrance to the infernal regions; hence the proverb, The descent to Avernus is easy, but coming back again is quite another matter, meaning that all bad habits are easily acquired, but very hard to be abandoned.