Meaning
The critical moment / The moment of truth
Example
Ele ia enfrentar o chefe sobre um aumento, mas na hora H, acovardou-se.
He was going to confront his boss about a raise, but at the moment of truth, he chickened out.
Origin
The expression is military and dates to World War I, not World War II. The U.S. Army borrowed the practice from the French: in operation orders the day an attack would begin was marked D-Day ("Dia D") and the exact start time H-Hour ("Hora H"), with the letters simply standing for "day" and "hour" so plans could be drawn up before the real date was fixed or kept secret.
Source: Wordorigins.org.
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More idioms
Acertar na moscaTo guess correctly / To hit the nail on the headAmigo da onçaA fake/false friendAndar na linhaTo toe the line / To behave properlyArmar um barracoTo cause a public sceneRodar a baianaTo cause a public scene / To go ballisticArrumar sarna pra se coçarTo ask for troubleArrastar as asasTo hit on someone / To flirtAbrir o jogoTo come clean / To open upArregaçar as mangasTo roll up one's sleeves / To get to workAgarrar com unhas e dentesTo hold on for dear life / To seize an opportunity