Tzafan means to hide, to store, to lay a thing away where it will be safe. It is used of treasure buried and of provision hoarded against a hard day. Not concealment in the sense of deceit; concealment in the sense of keeping.

"In my heart I have tzafanti your saying, that I might not sin against you" (Psalm 119:11). English usually gives "I have hidden" or "I have stored up," and both are accurate, and neither makes you feel the effort in it. He is doing the storing. He is carrying the words across a distance by the strength of his own love, hiding them where he hopes they will hold.

And storing is not inscription. A treasure hidden in a heart can be spent, or forgotten, or stolen in a weak hour. That is precisely the limit the new covenant answers: not a man storing the Torah inside himself by his own strength, but HaShem writing it into the grain of the heart, until it becomes the thing the heart actually wants.