In lyric writing, a dummy line is one that’s stuck into the rough draft of a lyric in lieu of a better solution. It keeps the song moving where it might otherwise stall.
A dummy line is temporary, and will often be tweaked or replaced entirely before the song is ready for performance.
Dummy lines ease writer’s block. A lyricist may find it easier to finish a lyric when she knows that certain lines are “only temporary,” and can be edited later. By definition a dummy line will be silly, cheesy, subpar, or even pure nonsense.
Dummy lines are also used sometimes to rough out a lyric melody for a track. The lyricist will then later go back and rewrite the fragmented or improvised lyric. The lyricist may scrap the dummy lyric entirely, or may find that the line actually works well enough to include in the finished song.