Dressed too casually for where you are. Everyone else is in a suit or a nice dress, and you came in jeans and a T-shirt — you are underdressed.

for where you are在你所在的場合，Dressed too casually穿得太隨便。Everyone else is in a suit or a nice dress別人都穿西裝或體面的洋裝，and you came in jeans and a T-shirt你卻穿牛仔褲和T恤來，you are underdressed你就是穿得不夠正式。

It is about the level of formality, not about the amount of cloth on your body, and this trips people up. A person in shorts on a hot beach is not underdressed; a person in shorts at a wedding is.

It is about the level of formality這是關於正式程度的問題，not about the amount of cloth on your body不是身上布料多少的問題，and this trips people up這一點常讓人搞混。A person in shorts on a hot beach在大熱天的沙灘上穿短褲的人，is not underdressed不算underdressed；a person in shorts at a wedding在婚禮上穿短褲的人，is就是。

The word almost always carries a feeling of mild embarrassment, and people usually use it about themselves: 'I felt so underdressed.' You will also hear the opposite, 'overdressed', when you are the only one in a suit and everyone else came in shorts — and in many cultures, including Taiwan, being overdressed feels safer than being underdressed.

The word almost always carries a feeling of mild embarrassment這個字幾乎總是帶著一點不好意思的感覺，and people usually use it about themselves而且人們通常是用來說自己：'I felt so underdressed.'就像『I felt so underdressed.』（我覺得自己穿得太隨便了）。You will also hear the opposite你也會聽到相反的說法，'overdressed'『overdressed』，when you are the only one in a suit and everyone else came in shorts當你是唯一穿西裝、其他人都穿短褲來的時候——and in many cultures, including Taiwan而在包括台灣在內的許多文化裡，being overdressed feels safer than being underdressed穿太正式感覺比穿太隨便安全。

Note that native speakers do sometimes use it loosely to mean 'not wearing enough clothes for the cold weather', though the formality meaning is the main one and the one dictionaries give.

Note that native speakers do sometimes use it loosely要注意的是，母語人士有時候會鬆散地用這個字，to mean 'not wearing enough clothes for the cold weather'來表示『不夠衣服禦寒』的意思，though the formality meaning is the main one不過正式度的意思才是主要的，and the one dictionaries give也是字典會給的意思。