2. Attended by favorable circumstances; propitious: "It seemed to be an auspicious time to ask for a raise in salary."
3. Promising success; propitious; opportune; favorable: "It was an auspicious occasion."
4. Favored by fortune; prosperous; fortunate.
Favorable omens came to be known as auspicious while unfavorable signs were considered inauspicious.
In Roman times, an augur was someone who foretold the future by observing the flight of birds (or by examining their entrails). Auspicium is divination (fortune telling) by observing the flights of birds and came from auspex, "someone who takes signs from the flight of birds".
A Latin derivative was the verb inaugurare "to foretell the future from the flight of birds", which was applied to the installation of someone in office after the appropriate omens had been determined; by the time it reached English as inaugurate, the association with the divination with birds had been forgotten.
An ancient Roman priest, or auspex, was appointed to foretell or divine the future outcome of an important event by observing the flights of birds, listening to their songs, observing the food they ate and by examining their internal organs.
Favorable omens came to be known as "auspicious" while unfavorable signs were considered to be "inauspicious".
Later the Roman auspex was replaced with the augur as the interpreter-observer of bird signs; his name being derived from the Latin avis, "bird", and garrire, "to talk" or "to tell". His interpretation, or augurism, became the English word augury, "an omen", and the Latin inaugurare, "to install an official after consulting the birds", became the word we use to install politicians in office with the hope that their "inaugurations" will prove to be auspicious for those who must endure their political machinations (plots and intrigues).
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An auspex interpreted the flight of birds as auspicious for the Romans to begin a new battle; however, he later divined that the birds had shown the situation to be inauspicious.