My mom sent me this one, so of course I had to look into it! I found the explanation pretty quick, but nevertheless enlightening, so let’s take a look!
Smithereens dates from 1810 and has always meant “small fragments.” No surprise there. But where does it come from? This is the interesting part. =) The smither part we know–it’s directly from the Irish Gaelic smidirin, which is itself a diminutive of smiodar, which means “fragment.”
So what about that -een? Is that where the “small” comes from? Etymologists can only take a good guess at that part, but their theory is that the -een was indeed applied as another diminutive, quoting names such as “Colleen” as evidence that it was done frequently in the Gaelic language. In my imagination I can see someone looking at minuscule fragments and deciding it was so small, it wasn’t just a smither, but a smithereen. 😉