Q. When an abbreviation is first mentioned in a footnote, should the abbreviation be spelled out in both the footnote and at the first mention of it in the body of the text, or is spelling it out in the footnote alone sufficient?
A. It can be perfectly acceptable to introduce an abbreviation in a footnote, but if you’re worried about readers who might skip the note, rework your text to define the abbreviation there rather than in the note. You’ll want to do this especially if the meaning of the abbreviation would be difficult to figure out without an explanation—or if you’re using endnotes rather than footnotes.
For an example of how you might use a note to introduce an abbreviation for the title of a frequently cited work (preferably in a footnote rather than in an endnote), see CMOS 14.59.