Play It by Ear

Literally a musical skill before it became a general one. To "play by ear" means performing a piece of music entirely from memory and listening, without ever reading it from written sheet music, a sense documented from the early 1800s, with some evidence suggesting the underlying idea goes back as far as the 1600s. The figurative sense, improvising your way through any situation rather than following a fixed plan, took hold from around the 1930s, borrowing the musician's trust in instinct and adaptation over rigid preparation.