"Button up your shirt"
What is the antonym of button up?
Is it button down?
Before taking off a shirt. We need to button ___.
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To "button up" a shirt implies that you start from the bottom button and work your way up. You can just say "button your shirt" and leave to the listener to decide how they prefer to do it.
A button-down shirt is one with an extra button for the collar.
This doesn't mean you start from the collar and work your way down (unless you want to). Rather the term "button-down" is likely short for "buttoned-down" meaning "securely fastened with a button".
The opposite of to button is, fortunately, to unbutton. Yes, it's one of those rare cases where English makes sense; however, you do not "unbutton down" a shirt. You just unbutton it.