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
The expression achar-se is the reflexive use of the verb achar ("to find" or "to think/consider"), so it literally means "to find oneself" — that is, to consider oneself something special.
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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