It comes from a song in Shakespeare's The Tempest (link).
Quote:
Full fathom five thy father lies,
Of his bones are coral made,
Those are pearls that were his eyes,
Nothing of him that doth fade,
But doth suffer a sea-change,
into something rich and strange,
Sea-nymphs hourly ring his knell,
Ding-dong.
Hark! now I hear them, ding-dong, bell.
[Fixed misspelling.]

Ceci n'est pas un seing.