What does the f stand for when looking at origin of a word on dictionary dot com?

The "f" you see in the etymology section of Dictionary.com stands for "from".

It indicates that the word in question comes from another language, and the word following "f" is the source language. For example:

* "f. Latin" means the word is derived from Latin.

* "f. Old English" means the word is derived from Old English.

This helps you understand the history and evolution of the word.