men are sometimes said to make women "weak in the knees" or "weak at
the knees."  In examining this, my friends and I have found that some
women literally - physically go weak in the knees and have to be held
up when their boyfriend or husband kisses them.  Why is this? And what
is the orgin of this phrase. We were shocked to find that it is
true....but why? We have searched everywhere and cannot find an
answer.

The origin of the phrase is very literal.  It is a fact that they do
actually become weak and their legs give out from under them, or
almost do.

The most common reason that a person becomes weak in the knees is
because their brain is not receiving enough blood.  It can preceed
fainting in some cases.  Lots of things cause fainting:
http://www.nhsdirect.com/articles/article.aspx?articleId=389
"Stressful situations, pain and excitement can all cause fainting."

The similarity between stressful, painful, or exciting situations and
kissing is that they all can cause the release of adrenaline into your
system and cause blood to be drawn away from your brain and toward
other parts of your body.
http://english.pravda.ru/science/19/94/377/12469_kiss.html
"In addition, kissing triggers massive adrenaline rush"

http://publicola.mu.nu/archives/2004/10/26/concealed_carry_for_a_paraplegic.html
"Adrenaline can make you go "weak in the knees"

The combination of adrenaline and increased blood flow to other areas
of the body can cause a person to have a decreased blood flow to the
brain.  Some degree of muscle control can be lost.  That will fist be
noticed in the muscle that is under the most stress, which would be
your leg muscles.

I hope that I have answered why a person can become "weak in the
knees" by being kissed.  If you require further information please
post a request for clarification.

On a personal note, I think some women fake the whole weak knees thing
to get their guy to hold them tighter.  But that's just my opinion. 
:)

Why should you be shocked?  You have said that it is a physical fact 
- which many of us can confirm.  A boyfriend's or husband's kiss can
be sexually arousing, and keeping knees straight and supportive
muscles tense becomes a minor concern in that situation, indeed,
counterproductive to her body's natural impulses, wanting him to
embrace her as closely as possible, etc., etc.