Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

crossjack
     n : the lowermost sail on a mizzenmast [syn: {mizzen course}]