This rare kanji has no clear standalone meaning. Its readings suggest laziness or neglect, but treat its meaning as uncertain.
The exact historical development of 寙 is uncertain. It visually contains 宀 (roof) and 㼌, but the original semantic and phonetic roles are unclear.
Under a roof 宀, a lazy person 㼌 (suggesting a melon-like shape, perhaps lounging) neglects work. The scene helps remember the idea of laziness.
For ユ, imagine a lazy person under the roof saying 'you' do it: you -> ユ, and the neglectful figure avoids work.