Play it by ear
Q From George: Trying to explain the phrase play it by ear is difficult to the Japanese, especially when they ask you why the word ear is used. What is the origin of this phrase?
A The phrase by ear goes back a long way in a figurative sense. It’s a metonym, the substitution of a word by another with which it is closely associated.
It’s in much the same style as Antony’s speech in Shakespeare’s Julius Caesar: “Friends, Romans, countrymen, lend me your ears”. He meant this figuratively, asking his audience to lend him the thing their ears contained, their function — in other words to listen to him, to hear him out. In phrases like by ear the process is taken one stage further: not merely the function of hearing but also being able to accurately reproduce a melody one has heard, without needing written music. So we have phrases like he has a good ear for music and she can play anything by ear.
The saying has been taken yet another step further away from anything literal when people use it to mean doing something in an extempore way, without planning, according to circumstances as they arise. If this is the sense in which your Japanese students encounter it, I’m not surprised that they find it puzzling.