I was asked the following question, and I thought it contained an important point in regards to Hebrew spelling:
Q: Why is the tefillin on the left arm? Is it in the Hebrew Bible or is it oral tradition?
A: It is an exegesis of the word ידכה Exodus 13:16.
וְהָיָה לְאוֹת עַל-יָדְכָה, וּלְטוֹטָפֹת בֵּין עֵינֶיךָ: כִּי בְּחֹזֶק יָד, הוֹצִיאָנוּ יְהוָה מִמִּצְרָיִם
“This will serve as a sign on your hand and as the front of a headband around your forehead that with a strong hand YHWH brought us out of Egypt.”
The word is usually written as ידך, which translates as “your hand”. The alternative pronominal suffix was more common in 2nd temple Hebrew, and in truth had no special meaning except for a variant of spelling due to spelling preferences at the time.
Second temple spelling tended to create alternative spellings based on adding א, ה, ו, י to express different vowels. This is similar to most of the spelling in the Tanakh, but was taken to an extreme where many consonants were added regardless of the actual core root of the word. We find additional consonants such as כיא instead of the regular form כי, and many more. These spellings in some cases are representations of pronunciations at the time (maybe הואה-Huwa instead of הוא-Hu), but in most cases they are vowel signs added by the writers. These additional consonants are not to be read into at all, and were a scribal practice to make reading easier.
However, in rabbinic thought every change has a meaning. In this case the thought was that this spelling is a compound word יד כהה which translates as “weak hand”. The idea is that the phylactery is to be placed on the non-dominant hand. This would mean that it would be mostly on the left hand, but with left handed people it would be on the right hand.