One can see the same (I suppose) എഴു in the following words as well:
- എഴുന്നു് - raised above the surface
- എഴുന്നേല്ക്കുക - to raise oneself up (either from a sitting or lying position)
Were we to consider the origins of writing - of scratching letters on palm leaves - we can get a clue. The letters "stood out" (എഴുന്നുനിന്നു) hence they were called എഴുത്തു്.
What about the components of writing? We have വര, കുറി (for lines, dashes), വള്ളി, പുള്ളി, കുനിപ്പു് (all for tittles) and കുത്തു് (for jot and full stop).
What about the letters themselves? അക്ഷരം is the word currently used to denote a (complete) letter. But അക്ഷരം obviously comes from Sanskrit (it actually means indestructible, indivisible, etc.). So are വര്ണ്ണം and കാരം. So what is the corresponding pure mallu word?
Or, is it that linguistics was formalized only with the advent of Sanskrit?
What about writing instruments? The original one would have been a നാരായം or എഴുത്താണി (in pure mallu). What about pen and ink? I don't think there are pure mallu words for these. പേന is most likely from "pen". മഷി is Sanskrit. മൈ is more mallu, but it looks almost like a തത്ഭവം from മഷി (മഷി -> മയി -> മൈ ).
What about paper? Originally they used പനയോല. Then came paper - കടലാസു്. I don't know the origin of the latter though.