When you have a butt injury or a sore butt and you are unable to make definite plans because you are unsure what the degree of pain will be like at the time of the plans.
"Thanks for inviting me to go to Yoga with you in the morning but I'm not sure, I'll have to play it by butt."
(note: can be applied to any body part i.e. play it by arm, play it by neck, play it by groin, etc.)
Letting you down politely, when you ask someone to do something and they want to do something, just not with you, they will say they're going to "play it by ear".
We say play it by "ear" because we are making decisions as we HEAR them. This means we are not making any final plans right now because we might hear of other things that could prevent us from being able to participate. So basically it means I might have bettershit to do.
Aimee: Hey can you hang out tomorrow night?
John: I might have to work late so let's play it by ear.