Basically it's a slang way of saying "I don't want to be stuck with Norma's part when we perform this/these song(s)

Another example of the use of the word: When I was a kid, "Bagsy not the back!" or "Bagsy the front!" were frequent cries before going anywhere in the car because everyone preffered to ride in the front seat so hope to bag the front seat by calling it first.

Teddy