*yeu-

The core idea of this root was:

youthful, with vigor

So it was basically about the energy that young people or beings have.
It’s one of the families that’s pretty much the same in most European Languages, be they Germanic, Roman or Slavic.
It’s the origin of words like young, youth, junior and juvenile.
The aspect of vigor is especially preserved in the German der Junge, which means boy and which probably revers to “boys being wild”.