let's go!, come on!
Bora lá, a festa já começou!
Let's go, the party has already started!
Bora is what fast, casual speech left of embora. Embora is itself a contraction of em boa hora ("at a good hour"), but alongside ir or vir it stopped carrying anything about time long ago and came to mean simply "away" — so vamos embora is "let's be off." Everyday Brazilian speech then wore the phrase down a step at a time, through the run-together forms vambora and simbora, until only bora was left.
Sources: Aulete — embora, escreva.ai — bora.
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