A capo is a capitalization typo, literally a misplaced capital or lowercase letter in a sentence. Example: "hello adam. wHat's The Commotion?"
TheMidnightFrogs coined the term on the second episode of their former podcast, Fatty Time. Yoshi said it on accident, but the portmanteau made sense, and it quickly entered their lexicon and is still used today.
Not to be confused with capo (pronounced "kay-poh"), which is a clamp used to raise the pitch of a guitar's strings.