In section 14 of my W-2 is the code "PA-CO1M." What does that code stand for?

It is one of the State of Pennsylvania mandatory deductions. You can categorize it as "other mandatory state or local tax not on above list" or you can select the option of "other - not on the list." 

Copy; thank you. What deduction is it?

It is most likely disability insurance. The code isn't defined. It shouldn't be an issue and won't affect your taxes.