Germanic Lexicon Project
The word f is only one character long. All one-character words are considered stopwords, i.e. high-frequency words which aren't indexed and can't be searched for.
f is not in the list of known abbreviations in Cleasby/Vigfusson. If it really is an abbreviation, please click here to report it.
Modern English: Search ARTFL's Gutenberg Webster for f
Modern German: Search Leo for f