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 image is H-Hour from WWI/WWII military planning. The allied forces used 'H-hour, D-day' to mark the precise moment an operation would begin — most famously, H-Hour on D-Day (6 June 1944).
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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