I agree that this is most likely a farm name. The origin of farm name and name of places is often old norse, - and this name might very well be that too. If you google gammelnorsk (old norse) and Hennum you may find several suggestions one of them is that it stems from the word "Heimr" (home), but I have not found anything "firm" about this.

But I am pretty certain about the norse origin. Good luck on your project

Britt

PS. the reason for different names in different families may be that up mid 1900's you might find Norwegian families with different family name. In my grandmothers family only the oldest son had the name "Karlsen" - "Karl's son", after his dad, Karl. So like in Norway it was usual to use the naming convention they use in Island to this day. The rest of my grandmother's siblings, inkluding herself got the surname "Larsen", which became the family name after the oldest son was born.

As a curiosity, I am named in the "church book" where my dad was born as "Britt Olafsdotter Fosseide" (daughter of Olaf, Fosseide is the name of the Farm), but that never was registered in the National name registry in Norway.

Hope this helps :)