From
Aeneid (book 6-th, lines 236÷242):
| Spelunca
alta fuit vastoque inmanis hiatu,
Scrupea, tuta lacu nigro nemorumque tenebris, Quam super haut ullae poterant inpune volantes Tendere iter pinnis: talis sese halitus atris Faucibus effundens supera ad convexa ferebat. [Unde locum Grai dixerunt nomine Aornon] |
There
was a deep cavern, enormous for its wide mouth,
harsh, protected by a black lake and the darkness of the wood. No bird could, over it, without damage, prolong its fly; such a bad breath, out of the dark fauces, was ascending to the vault. [that's why the Greeks called the place "Aornon"]. |
One might thus infer that the word "Aornos" is derived from Greek:
privative alpha + 
(that is, without birds).
Anyway, the last line above is an apocryphal interpolation not belonging to Virgil. The word instead would take origin from the Latin word "Avernus", the lake near Cuma (Campania, Italy), famous for the presence of the Sibyl's oracle.