Meaning
To be full of oneself / To act high and mighty
Example
Desde que foi promovida, ela anda com o rei na barriga.
Since she got promoted, she's been acting high and mighty.
Origin
The expression comes from the age of monarchy. When a queen — or a royal favourite — was pregnant by the king, she was treated with special deference, since she might secure the dynasty and an heir to the throne. To "have the king in one's belly" literally described that privileged condition.
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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