The Collaborative International Dictionary of English v.0.48:

Crossjack \Cross"jack`\ (kr?s"j?k` or kr?"j?k`), n. (Naut.)
   The lowest square sail, or the lower yard of the mizzenmast.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crossjack
    n 1: the lowermost sail on a mizzenmast [syn: crossjack,
         mizzen course]