Avernus

prefix meaning "not, without," from Greek a-, an- "not" (the "alpha privative"), from PIE root *ne- "not" (source also of English un-).

In words from Greek, such as abysmal, adamant, amethyst; also partly nativized as a prefix of negation (asexual, amoral, agnostic). The ancient alpha privatum, denoting want or absence.

Greek also had an alpha copulativum, a- or ha-, expressing union or likeness, which is the a- expressing "together" in acolyte, acoustic, Adelphi, etc. It is from PIE root *sem- (1) "one; as one, together with."

before vowels ornith-, word-forming element meaning "bird, birds," from combining form of Greek ornis (genitive ornithos; plural ornithēs) "a bird," in Attic generally "domestic fowl, cock or hen," which often was added to the specific name of the type of bird, from PIE *or- "large bird" (see erne).

For "bird" Greek also had ptēnon (plural peteina), related to pteron "wing," from the root meaning "to fly."