1. [n]  position 40 in a countable series of things
2. [adj]  the ordinal number of forty in counting order

FORTIETH is a 8 letter word that starts with F.

1. \For"ti*eth\, a. [AS. fe['o]wertigo?a. See {Forty}.]
   1. Following the thirty-ninth, or preceded by thirty-nine
      units, things, or parts.
   
   2. Constituting one of forty equal parts into which anything
      is divided.
   
   

2. \For"ti*eth\, n.
   One of forty equal parts into which one whole is divided; the
   quotient of a unit divided by forty; one next in order after
   the thirty-ninth.