Source : Webster's Revised Unabridged Dictionary (1913)

Fortieth \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.

Fortieth \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.

Source : WordNet®

fortieth
     adj : the ordinal number of forty in counting order [syn: {40th}]
     n : position 40 in a countable series of things