I'd gander a guess that it's there for FAT, File Allocation Table. But if you look at Wikipedia the "f" stands for "fixed" as in "fixed disks".
excerpt - http://en.wikipedia.org/wiki/Fdisk
For computer file systems, fdisk (for "fixed disk") is a command-line utility that provides disk partitioning functions. In versions of the Windows NT operating system line from Windows 2000 onwards, fdisk is replaced by more advanced tool called diskpart. Similar utilities exist for Unix-like systems.
Granted the above has more to do with the Windows/DOS variant but the term "fixed disk" makes a lot of sense, since hard drives were often termed "fixed" in the olden days.
"fixed disk" definition
The definition of "fixed disk" also says the same.
excerpt - http://www.thefreedictionary.com/fixed+disk
Noun 1. fixed disk - a rigid magnetic disk mounted permanently in a drive unit
Other sources saying the same thing:
Original origins of "fixed disk"
Wikipedia's page on Hard Disk Drives also had this nugget:
In 1961 IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called fixed disk drives.