I'm not sure if it's a commonly used reference, because I seldom hear it in other artists' songs. (may also be because songs by other artists are more about yay yay happy love most of the time
) It's also too romantisized to be used in common-day talk too, so you won't hear it in normal everyday conversations. So I'm not sure if it's a popular concept that everyone knows of.
And yes, she uses the word senaka (背中), which literally means back (the body part). So the meaning is probably more poetic than anything.
