the odd one out idiom UK (also the odd man out)
与众不同的人（或物）；异类；不合群的人
a person or thing that is different from or kept apart from others that form a group or set
Guess which number of the following sequence is the odd one out. 猜猜看，以下一系列数字中哪个与其他的不一样。
She was always the odd one out at school - she didn't have many friends. 她在学校里总是形单影只——她没有多少朋友。