> the characters got arbitrarily attached to an existing word because they were similar in sound and meaning

It does not seem arbitrary in this case because the meaning does match.

I do take your point that using that word in the discussion above, which is about Japanese was not the best example. On the other hand, it is a good example in chinese.

(A) In Japanese the meanings don't match that closely. The word didn't originally mean cute, but rather pathetic or pitiable, and evolved over time. More info: http://gogen-allguide.com/ka/kawaii.html

(B) By arbitrary here I mean that there is no linguistic connection. "Arbitrarily chosen because they are similar" => "chosen for no reason other than their similarity".