We'll play it by ear?

Question

Is it possible that when the term was made up it was, "We'll play it by year" (which seems far more practical), but then interpreted as, We'll play it by ear?

Answer 1

No, the idea was musical:  we don't have the written music, so we'll guess at the notes according to how they sound on our instruments(we'll hear the sound we play and decide if it's the music we want to hear).  Now of course the phrase is said when anything is done without written instructions or plans.  

Answer 2

I don't think so, although it's an interesting theory. I'm not quite sure what "play it by year" would mean, though.

Musicians play by ear when they do not follow written music but simply play a piece as they remember hearing it. If they've forgotten a section, they may improvise briefly, and their rendition will almost certainly include mistakes or variations from the original.

Acting without a specific plan is similar, hence the use of the term.