A more thorough explanation:

Definition: F. has two meanings:

1. It is an abbreviation for the first series of the Federal Reporter. This series includes federal decisions (trial and appellate) from 1880 to 1924.
2. In history, F. was a letter branded on a felon who claimed benefit of clergy. This meant that the felon could claim the benefit only once. Additionally, those convicted for an affray (fray) or falsity were also branded with the letter F. The branding was done with a hot iron on the cheek or ear of the felon. The letter F was used to identify the felon as a fray-maker or fighter.

For example, Thomas Blount's Nomo-Lexicon: A Law-Dictionary (1670) states, "He that shall maliciously strike any person with a Weapon in Church or Churchyard, or draw any Weapon there with intent to strike, shall have one of his Ears cut off; and, if he have no Ears, then shall be marked on the Cheek with a hot Iron, having the Letter F, whereby he may be known for a Fray-maker or Fighter."

This example illustrates how the letter F was used to brand felons who committed violent crimes. The branding was a form of punishment and also served as a way to identify the felon as a repeat offender.

Giles Jacob's A New Law-Dictionary (8th ed. 1762) also explains the use of the letter F. Jacob writes, "F, Is a Letter wherewith Felons, &c. are branded and marked with an hot Iron, on their being admitted to the Benefit of Clergy."

This example shows that the branding with the letter F was done when a felon claimed benefit of clergy. Benefit of clergy was a legal concept that allowed first-time offenders to avoid the death penalty by claiming that they were clerics and therefore subject to ecclesiastical law instead of secular law.