Meaning:
“花蟹,” literally “swimming crab,” is a marine crab with some legs flattened and fringed for swimming. Nowadays, netizens use this term to refer to a person who always tries to obstruct others from doing things, even though he or she receives no benefit from such obstruction or harm. This use comes from a South Korean reality show, “New Journey to the West,” in which the MC Kang Ho-dong describes the fact that when you boil swimming crabs, you don’t need to cover the boiling pot, because each swimming crab always tries to drag down other crabs that try to climb out, thus preventing them from escaping.
Example:
A: 你不是去教室准备明天的考试吗？怎么这么早就回来了？
Nǐ búshì qù jiāoshì zhǔnbèi míngtiān de kǎoshì ma ？zěnme zhè me zǎo jiù huílái le ？
Didn’t you go to the classroom and prepare for tomorrow’s exam? Why are you home so early?
B: 我同学小明心情不好，所以他一直拉着我聊天，我就只好回来学。
Wǒ tóngxué xiǎomíng xīnqíng bú hǎo ，suǒyǐ tā yīzhí lāzhe wǒ liáotiān，wǒ jiù zhīhǎo huílái xué 。
Xiaoming, my classmate, was in a bad mood, so he kept chatting with me. I have to come back and study at home.
A: 他可真是花蟹啊。
Tā kě zhēnshì huāxiè ā 。
He really obstructs other people without benefiting himself.