Burden of a Song
The words repeated in each verse, the chorus or refrain. It is the French bourdon, the big drone of a bagpipe, or double-diapason of an organ, used in fortë parts and choruses.
“The burden of proof is on the party holding the affirmative” [because no one can prove a negative, except by reductio ad absurdum ]. —Greenleaf: On Evidence (vol. i. part 2, chap. iii. p. 105).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894