「いやだ」は、何かをしたくない時や、好きじゃない時に使う言葉です。

"いやだ" is a word used when you don't want to do something or don't like it.

子供も大人も、気持ちを強く伝えたい時によく言います。

Both children and adults often say "いやだ" when they want to strongly express their feelings.