1. [n]  position 50 in a countable series of things
2. [adj]  the ordinal number of fifty in counting order

FIFTIETH is a 8 letter word that starts with F.

1. \Fif"ti*eth\, a. [AS. f[=i]ftigo[eth]a. See {Fifty}.]
   1. Next in order after the forty-ninth; -- the ordinal of
      fifty.
   
   2. Consisting of one of fifty equal parts or divisions.
   
   

2. \Fif"ti*eth\, n.
   One of fifty equal parts; the quotient of a unit divided by
   fifty.