Informal:
* 戇居 (ngong gwai): This is the most common and informal way to say "silly." It literally translates to "stupid" but is often used in a playful or endearing way.
* 傻 (sa): This is another informal way to say "silly" and is often used to describe someone who is naive or easily tricked.
Formal:
* 愚蠢 (jyu chuen): This is the more formal and polite way to say "silly," and is often used in writing or when speaking to someone you respect.
Other options:
* 笨 (bun): This means "stupid" and can be used to describe someone who is slow or lacking in intelligence.
* 蠢 (chuen): Similar to "bun," this word also means "stupid" and can be used to describe someone who is foolish or lacking in common sense.
The best way to choose the right word depends on the specific situation and your relationship with the person you are talking to.