阴雨天u
[yīn yŭ tiān u]
The meaning is 'On a rainy day'. Some Chinese netizens like to use the letter u to replace 'you', because in Chinese pinyin, 'u' sounds like 'you'. So, this nickname might mean that the person likes or has some unforgettable memories of rainy days, and they also wish someone to accompany them during the rain.