The Collaborative International Dictionary of English v.0.48:

Noonday \Noon"day`\, n.
   Midday; twelve o'clock in the day; noon.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Noonday \Noon"day`\, a.
   Of or pertaining to midday; meridional; as, the noonday heat;
   only mad dogs and Englishmen go out in the noonday sun.
   "Noonday walks." --Addison.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

noonday
    n 1: the middle of the day [syn: noon, twelve noon, high
         noon, midday, noonday, noontide]