Able to be taken in without strain. Readability is a property of the writing, never of the reader.
Built on read, from Old English rædan (to interpret) + the suffix -able; a readable thing is one that has been made to yield its sense without a fight.
Able to be taken in without strain. The root keeps the burden on the writing rather than the reader — readability is a property of the text, never a demand made of the person reading it.