不会游泳的鱼
[bù huì yóu yŏng de yú]
It stands for 'A fish that cannot swim'. Since all fish can normally swim very well, this nickname is quite novel. Perhaps the person using it feels they have certain shortcomings or limitations in real life but want to show off their quirky characteristics, thus making them appear more distinctive and full of personality by naming themselves after an 'abnormal' thing like a fish that doesn't know how to swim.