The core idea of this root was:

orphan, without father, “change allegiance”

And very early on, people also started using it in the sense of being poor or in serfdom, because that was a common fate for orphans.
This idea is reflected in the German arm (poor).
And also the word Arbeit used to means hard, tiring labor – the kind that serfs would have to do.

The German erben, which is about inheriting, on the other hand ties in with the idea of having lost the father.

Here’s an overview over the English relatives:

• robot (a slave like worker)
• orphan, orphanage