Play it by Ear

Etymology:

• It is a 17th century phrase which meant to remembering or playing the song just by knowing the melody, or having the song in your mind which did not require notes to present it. But three centuries later, it meant to improvise things without certain future plan. Suppose you go to a new place without knowing about it. There you play it by ear, as you get there, then decide where to stay and what to do, hence in emergency situations people have to play it by ear.