Native
Dutch person here, born and raised.
Assuming that OP's great-grandfather was an immigrant to the United States, Johun is in all likelihood a misspelling (or transcription error) of either
Johan or
Jochem (also
Jochen), thanks to American government officials and/or clerks being unfamiliar with
Dutch given names and the
Dutch language. There is no way that Johun could have been a legitimate but obscure variant of
Johan or
Jochem in the Netherlands: the variant does not make sense linguistically and there is also the fact that
Dutch people were not particularly creative with given names in the old days and made sure to stick with known and tried-and-true variants (partly out of societal reasons and partly out of legal reasons, as the Netherlands have had a naming law since at least the Napoleonic era).
Johun not being a legitimate
Dutch given name is further backed up by the fact that Johun is not featured in this academic database of
Dutch given names:
http://www.meertens.knaw.nl/nvb/english (in English)
-
TerHorst belongs in the surname board, but is a southern dialect/language version of the German phrase "der Horst"
No, Ter
Horst is a legitimate
Dutch surname and therefore not necessarily a translation of a
German one (or otherwise originating from Germany). Also,
ter is
not the
Dutch equivalent of
German der: it is actually a (mostly dated)
Dutch word that (depending on context) can mean "on" as well as "in" and "at". For example, there is the
Dutch phrase "Ter land, ter zee en in de lucht" which means "On land, at sea and in the air" in English. Also see this article on Wiktionary:
https://en.wiktionary.org/wiki/ter#Dutch (in English)
As for
horst (plural is
horsten): this is etymologically related to what you mentioned, but it has a slightly different meaning in
Dutch. In
Dutch,
horst is a historical term for an elevated piece of land that is overgrown by brushwood or used for coppicing.
-
https://en.wiktionary.org/wiki/horst#Dutch (in English)
-
https://nl.wikipedia.org/wiki/-horst (in
Dutch)
So, all in all, Ter
Horst is a
Dutch toponymic surname that indicates that the original bearer either lived near a horst or managed a horst. The surname is most common in the east of the Netherlands:
http://www.cbgfamilienamen.nl/nfb/detail_naam.php?gba_lcnaam=ter%20horst&gba_naam=ter%20Horst&nfd_naam=Horst,%20ter&operator=eq&taal= (in
Dutch)
Its variant Terhorst (which would
never be spelled as terHorst in the Netherlands - that is an americanized spelling) is less common, but also predominantly found in the east:
http://www.cbgfamilienamen.nl/nfb/detail_naam.php?nfd_naam=Terhorst&info=aantal%20en%20verspreiding&operator=eq&taal= (in
Dutch)
Last but not least, the best known
Dutch bearer of the surname is Guusje ter
Horst (b. 1952), a former minister of Interior and Kingdom Relations:
https://en.wikipedia.org/wiki/Guusje_ter_Horst (in English)
"How do you pick up the threads of an old life? How do you go on... when in your heart you begin to understand... there is no going back? There are some things that time cannot mend... some hurts that go too deep... that have taken hold." ~
Frodo Baggins