[chki]

I think that is not only incorrect considering the content but probably also the semantics involved. There are many German words that have "Unter" in them ("Unternehmen" --> Company; "Untersuchung" --> study) where it does not relate to the word "unten" in any way.

[kgwgk]

In fact it seems to be related to the "among" meaning of "unter". The etymology is similar for other languages:

http://www.etymonline.com/index.php?term=entertain