Meaning
To show off / To seek attention
Example
Rafael não para de falar sobre seu carro novo caríssimo, se quer aparecer, que coloque uma melancia na cabeça!
Rafael won't stop talking about his expensive new car. If he wants to show off, he might as well wear a watermelon on his head!
Origin
The expression colocar melancia na cabeça ("to put a watermelon on one's head") rests on a piece of comic folk imagery rather than a documented historical event: someone walking around with a watermelon balanced on their head simply cannot go unnoticed.
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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