1. to improvise, to do something unplanned or unexpected, without preparation
do/change it on the fly
To do something 'on the fly' is to do something in a way that isn't planned. Similar in meaning to 'improvise' or 'make it up as you go along'. It can also mean to do something or change plans while you are in motion or while something is already in progress. "We didn't plan our route, we just drove along the coast and stopped in different places on the fly." "My grandmother is a great cook, she never follows recipes she just adds things on the fly as she goes." This is an idiomatic expression used more often in social contexts.