The short answer is that it's the English translation for the Irish word smidirin. The longer answer is that most Indo-European languages have 'smith' or 'smitha' as a worker with a hammer, or the object of such work. Thus, a 'smitha' (cognate with 'just a smidge') can be an object or piece of an object, and the addition of the Irish diminutive 'een' makes it a small fragment of an object.