"Knees-up" now refers to a lively party in England, often a dance party. The term comes from this song by Harris Weston and Bert Lee written in 1939. It's not as popular as a term now... yet the song is still sung in pubs in England, especially the second version below.

Notes

Common Modern Lyrics in the UK:

Knees up Mother Brown!
Knees up Mother Brown!
Under the table you must go
E-I-E-I-E-I-O!
If I catch you bending,
I'll saw your legs right off*,
Knees up! Knees Up!
Don't get the breeze up,
Knees up Mother Brown!

Oh My! What a rotten song!
What a rotten song!
Oh, What a rotten song!
Oh My! What a rotten song!
What a rotten singer too!

Knees up Mother Brown!
Knees up Mother Brown!
Under the table you must go
E-I-E-I-E-I-O!
If I catch you bending,
I'll saw your legs right off,
Knees up! Knees Up!
Don't get the breeze up,
Knees - up - Mother - Brown!
Ow's yer farver**? All right!

*Or "We'll turn you upside-down".
**Literally, "How's your father?" It was a catch phrase in the music halls in Britain and then during WWII. It has a certain innuendo.

*****
Lesley wrote:

"With regard to the song 'Knees up Mother Brown'.

My father George was a true cockney born within the sound of bow bells.

There was an introduction to the 'Knees up mother Brown' song that the Londoners used to sing. It goes like this.

I went to a ding dong down old Brixton way where Mother Brown, the pearly Queen was a 100 years that day. Oh what a celebration, oh what a la de da when someone rolled the carpet up and shouted 'come on ma'. .....then it starts ......Knees up Mother Brown, knees up Mother Brown etc.

Kind regards

Lesley"
*****
According to Wikipedia:

There also exists a version of the song for children, with accompanying dance. The lyrics are :

There came a girl from France
Who didn't know how to dance
The only thing that she could do was
Knees up Mother Brown.

Oh, knees up Mother Brown
Knees up Mother Brown
Knees up, knees up, never let the breeze up
Knees up Mother Brown.

Oh, hopping on one foot
Hopping on one foot
Hopping, hopping, never stopping
Hopping on one foot.

Oh, knees up Mother Brown
Knees up Mother Brown
Knees up, knees up, never let the breeze up
Knees up Mother Brown.

Oh, hopping on the other
Hopping on the other
Hopping, hopping, never stopping
Hopping on the other.

And whirling round and round
Whirling round and round
Whirling, whirling, never twirling
Whirling round and round.

Thanks and Acknowledgements

Thanks to Lesley for sharing another verse!