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 image is of a queen pregnant with the next king — 'ter o rei na barriga' describes a person who carries themselves as if they were royalty.
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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