The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Constituting one of forty equal parts into which anything
      is divided.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fortieth
    adj 1: the ordinal number of forty in counting order [syn:
           fortieth, 40th]
    n 1: position 40 in a countable series of things