Simpson and Roud in their authoritative Oxford Dictionary of English Folklore (2000) indicate that the earliest reference to breaking a mirror meaning bad luck dates to 1777. "Earliest reference" does not indicate "origin" since folk traditions notoriously float around in oral tradition before being recorded. This merely means that it was first recorded in 1777 - and it has appeared in the written record consistently after that. Because beliefs and traditions about mirrors apparently are very old, it is possible that the concern over breaking a mirror is also far older than the eighteenth century; Simpson and Roud note that "many of the [mirror-related traditions] can be shown to date back a long time."

Simpson and Roud also indicate that bad luck or even death was apparently the original tradition; the "7 years" was first recorded in the mid-nineteenth century. They also point out that there were early precautions associated with mirrors including not allowing a baby to see itself in a mirror and covering mirrors when someone had died, for fear of seeing a reflection of the ghost who might be lingering.

In this concern, there is likely to be something of an explanation for the origin of the motif (although it is always risky to take these explanations too far): the mirror is a reflection of the world - including the person looking into the mirror; to see that world (and the image of ourselves), we are clearly not seeing the world, (or ourselves), but rather what appears to be some otherworldly version of our surrounds. It was easy for the folk mind to consider this to be a means to view our souls, and to shatter a mirror could, by deduction, mean that the person who caused the accident would have a damaged soul.

This does not, however, cast light on the origin of the folk belief except to give a speculative considerations of the why and a poor attempt at the when. The breaking part of this is clearly linked to the use of glass for mirrors, but since glass has been used in this way for a long time, that does not help much. I suppose we could focus on the widespread use of glass for mirrors, and that likely takes us to a time not before the late medieval period.