The lyrics were added by Sunny Skylar AFTER the record became a massive chart hit for Glenn Miller in 1944, two years after it was recorded. It went all the way to no. 2 on the Billboard Harlem Hit Parade Chart, the then equivalent of the later R&B Chart. It went to no. 16 on the Billboard Juke Box Chart also in 1944. Woody Herman and Johnny Long covered it. On the original Glenn Miller RCA record label, Skylar is not listed as a co-composer. Sunny Skylar wrote the expanded lyrics to the song that are heard in later versions such as the Herman recording. It is much like what happened with Moonlight Serenade with Mitchell Parish later adding lyrics to what was initially an instrumental. The saying "It Must Be Jelly 'Cause Jam Don't Shake Like That" sounds like a traditional slang expression that goes back a long way. It is similar to the Duke Ellington title "Don't Do Anything Until You Hear From Me" which he did not create, but which is an expression that white society used. So Duke was just mimicking an expression he heard from the larger white society. This is the same way the Glenn Miller expression was picked up.