Possible Clues:

• Cousin of ibid
• Bibliography notation
• Relative of ibid
• Footnote kin of ibid
• Dissertation abbreviation
• In the work already mentioned: Abbr
• In the work mentioned (Abbr.)
• Note I put on page in book (it's one I mentioned before)
• Ref. to an earlier ref
• 'In the work cited' (Abbr.)