Brit-Speak Help Again: "Knees up" >??

[Catlady (Rita Prince Winston) <catlady@wicca.net>]

*hugs Saitaina whil blushing*

--- In HPFGU-OTChatter at yahoogroups.com, "Steve <bboy_mn at y...>" 
<bboy_mn at y...> wrote:

> Can anybody help me with the phrase "kneed up"?

OT was discussing on-line dictionaries recently. I love 
http://www.onelook.com/index.html -- it checks many on-line 
dictionaries for you, including the deservedly praised dictionary.com 
-- and including specialised and slang dictionaries. This time it 
found me:

http://www.peevish.co.uk/slang/k.htm
"Knees Up. Noun. A lively party. {Informal}. E.g."There's going to be 
a knees-up at Jenny's tonight, it's her birthday." >>

and

http://dictionary.cambridge.org/define.asp?key=knee*1+8
"(British informal dated or humorous) A knees-up is an energetic 
noisy party where people dance."

I had never before heard any implication that a knees-up must include 
dancing, but that would explain the phrase: the knees go up because 
the dance is the Charleston or some such.