My favorite meaning of umbra is one I read once that attributed Umbra as a term in ancient Rome for "an uninvited guest". (I cannot find the dictionary I got this out of now though, to note it in full) This was used if an invited guest invited a third person to a party, that person was welcomed, but as an umbra.
In some ways I find it scary that the Romans had a term for this.
Umbra is also the name of my odd dog, in that he is black (penumbra) and was a dumped/stray animal that we adopted (uninvited guest).