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.)
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