It is an autocorrect entry.  In Excel 2003 or earlier, click on Tools |
AutoCorrect Options | AutoCorrect tab, scroll down the list of
autocorrect entries until you see the entry for "adn", click on it once
to select it, and click on Delete to delete it.  Note that your other
Office applications share this autocorrect list, so deleting this entry
from Excel also deletes it from Word, Powerpoint, etc.