To resist; to keep (someone or something) from coming near.
'Fend off' is one of the numerous little idioms in English that make our language rich and are such a burden to people learning it as a second language. 'Fend' - what is that exactly? There's no equivalent 'fend on'.
As is so often the case, 'fend' is a word that, although it has ceased to live as a word in its own right, has stayed on life-support by virtue of the expressions 'fend off' and 'fend for oneself'.
To go back to when 'fend' was in active use we need to relocate to the 14th century. The Old English texts from then use 'fend' as an alternative to 'defend'. So, if we make that substitution in the above expressions we can get an understanding of how they came about. Strangely, 'fend off' wasn't used in the 14th century, in fact, not until the 19th century when 'fending' was long since defunct. The first example that I have found of the expression in print is in the English Sunday newspaper The Examiner, 1830:
A Committee is the fend-off to importunity, and the contrivance for obtaining time.