Going back to 1918, a popular bar song in London was “Knees up, Mother Brown”. Finally recorded in 1938, it was quite popular for a number of years. In this clip from Hullabaloo, we can hear Petula Clark and Noel Harrison sing it with a cockney accent in a duet. As it was preceded with a snippet of “I’m Henry the 8th I am”, I assume it is from around 1966.
Fozzie the bear sang it in 1980 on the Muppet Show, and Michael Caine in 1986 in “Sweet Liberty”.
Given that it was done on American television, it must have been “clean” enough in context by that time.
However, the same article claims that the phrase originally had a different, earthier sense (Read sexual). I can find no supporting documentation for that statement, but it seems obvious.
Can anyone nail down the original first usage of this phrase, or is it possible it was only an oral tradition?