爱多久不出轨的小火车不会走不劈腿的美人鱼
[ài duō jiŭ bù chū guĭ de xiăo huŏ chē bù huì zŏu bù pī tuĭ de mĕi rén yú]
This playful, whimsical nickname combines two metaphors to create a surreal image. The first part 'a little train that never strays off track' implies staying true and faithful in relationships; the second, 'a mermaid who won't cheat', emphasizes faithfulness in romantic contexts.