Jamieson, Fausset and Brown on Exodus 13:9

9. it shall be for a sign unto thee upon thine hand, &c.--There is no reason to believe that the Oriental tattooing--the custom of staining the hands with the powder of Hennah, as Eastern females now do--is here referred to. Nor is it probable that either this practice or the phylacteries of the Pharisees--parchment scrolls, which were worn on their wrists and foreheads--had so early an existence. The words are to be considered only as a figurative mode of expression.

that the Lord's law may be in thy mouth, &c.--that is, that it may be the subject of frequent conversation and familiar knowledge among the people.