The Collaborative International Dictionary of English v.0.48:

F \F\ ([e^]f).
   1. F is the sixth letter of the English alphabet, and a
      nonvocal consonant. Its form and sound are from the Latin.
      The Latin borrowed the form from the Greek digamma ?,
      which probably had the value of English w consonant. The
      form and value of Greek letter came from the Ph[oe]nician,
      the ultimate source being probably Egyptian.
      Etymologically f is most closely related to p, k, v, and
      b; as in E. five, Gr. pe`nte; E. wolf, L. lupus, Gr.
      ly`kos; E. fox, vixen; fragile, break; fruit, brook, v.
      t.; E. bear, L. ferre. See Guide to Pronunciation,
      [sect][sect] 178, 179, 188, 198, 230.
      [1913 Webster]

   2. (Mus.) The name of the fourth tone of the model scale, or
      scale of C. F sharp (F [sharp]) is a tone intermediate
      between F and G.
      [1913 Webster]

   F clef, the bass clef. See under Clef.
      [1913 Webster]

WordNet (r) 3.0 (2006):

F
    n 1: a degree on the Fahrenheit scale of temperature [syn:
         degree Fahrenheit, F]
    2: a nonmetallic univalent element belonging to the halogens;
       usually a yellow irritating toxic flammable gas; a powerful
       oxidizing agent; recovered from fluorite or cryolite or
       fluorapatite [syn: fluorine, F, atomic number 9]
    3: the capacitance of a capacitor that has an equal and opposite
       charge of 1 coulomb on each plate and a voltage difference of
       1 volt between the plates [syn: farad, F]
    4: the 6th letter of the Roman alphabet [syn: F, f]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

F, punishment, English law. Formerly felons were branded and marked with a
hot iron, with this letter, on being admitted to the benefit of clergy.