element

be in your element

to feel happy and relaxed because you are doing something that you like doing and are good at. You should have seen her when they asked her to sing, she was in her element.

be out of your element

to feel unhappy or strange because you are in a situation that you are not familiar with. He felt out of his element at such a formal occasion.

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in your element

doing something you like very much and are comfortable with. Paul is in his element when he's working in the office, but when he's making sales calls, he's miserable.

Usage notes: the opposite meaning is expressed by out of your element doing something that makes you very uncomfortable or unhappy: Grant was a great general, but as president, he was out of his element.