The title of "Imperator" was an old Roman honourary title I think. IIRC Scipio Africanus was called Imperator as a term of respect for his accomplishments on the battlefield...
As you say Augustus chose the title and gave it a new meaning. Mainly because the Romans hated Kingship (stemming from the fact that their early kings were brutal tyrants who deserved to be overthrown) They don't follow kings, they kill them.
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