Meaning
To be full of oneself / To think too highly of oneself
Example
Aquela garota se acha só porque apareceu na televisão.
That girl is full of herself just because she appeared on television.
Origin
Literal: 'to find oneself' = to think too much of oneself. The reflexive pronoun gives the phrase the ironic sense of self-inflation.
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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