What is the か in「か弱い」?


I'm wondering what the か in か弱い, か細い and similar words is. It seems to act as
an intensifier. The Daijisen tells me simply that this か is a 接頭語, and
translates it as いかにも; it doesn't seem to have its own entry or any further
explanation. Speculations online seem to include:

  * it is a corruption of 小【こ】, and so is a simple intensifier
  * it implies "obviously 弱い・細い _by sight_ "

My question: what does this prefix mean, and where does it come from? (For
bonus points: can anyone give me some more examples, and is this prefix still
productive?)




The best answer you might get with this is that it's "just a prefix." I cannot
find any indications of the origins, but it is included in
[dictionaries](http://dic.yahoo.co.jp/dsearch?enc=UTF-8&p=%E3%81%8B&dtype=0&dname=0na&stype=1&pagenum=21&index=02709300)
and defined simply as a prefix that strengthens meaning:

> ［接頭］主として形容詞に付いて、意味を強め、語調を整える。「―弱い」「―細い」「―黒い」

I can only find the one page with the theory that it comes from 小 so I don't
know how much validity we can give it. It's probably just an old prefix that
fell out of use but remains as a relic in a few set words.

I welcome anyone who can prove me wrong, though.

