Casual clothes are not formal or not suitable for special occasions.

He was dressed in casual clothes--sneakers, blue jeans, and a t-shirt.

not taking or not seeming to take much interest

The psychologist's attitude seemed far too casual, even brutal. 那个心理学家的态度似乎太过冷淡了，甚至有些冷酷。

Security around the conference hotel seemed almost casual. 举行会议的酒店周围的安全措施似乎不太严格。

Although close to tears, she tried to make her voice sound casual. 尽管都快掉眼泪了，可她还是尽力使自己的声音听上去满不在乎。

not regular or fixed

casual sex They were having casual sex but it never became a serious relationship.

casual labour/labourers/workers 零工／临时工

not serious or considered, or done by chance

casual observer To a casual observer, everything might appear normal. 对于一个漫不经心的人来说，也许看上去一切正常。

It was just a casual comment - I didn't mean it to be taken so seriously. 我只是随便说说而已，不是当真的。