Is there anybody out there?
"Mutterseelenallein" is the 'mother' of all German words for 'loneliness.'
Even though the word sounds like a typical German word - or rather three German words pasted together into one long German adjective - the term actually derived from the French idiom "moi tout seul," meaning "me all alone."
When the Huguenots, a group of persecuted French Protestants, fled to Germany in the 18th century, they used the term "moi tout seul" to describe their feeling of isolation and dislocation from home. But the Germans misunderstood the phrase as "mutterseelen" (mother's souls) and added the word "allein" (alone) so that the phrase would make sense to them.
When you're "mother's souls alone," as this German adjective literally translates to, there is neither your mother nor any other soul around you. When you're "mutterseelenallein" you're not just alone, you're completely, utterly, alone.
While being all by yourself can be great - picture yourself on a lonely beach with just your book, a hammock and a delicious cocktail - "mutterseelenallein" usually has a more existential, negative connotation and can be used as a synonym for isolated, abandoned, or desperate.
If you don't want to be the only one who is "mutterseelenallein" for not knowing this term, then you'd better make a note of it now!