Stephenson:Neal:Quicksilver:383:Merry madrigal tunes (Mcwee)

From the Quicksilver Metaweb.

It is rumored that, originally, the "fa la la"s were inserted in popular madrigals to mask vulgar (by which I mean to say sexually explicit or entendre-filled) passages, too saucy for polite society or to be commited to paper. Later, the "fa la la"ing caught on in its own right, as an artistic device. Frankly, the odds of Jack singing a salacious song seem slightly higher than the odds of him singing a non-salacious song.