Definition of jackcrosstree in English:

jackcrosstree

noun

historical, rare Nautical
  • Either of the highest pair of crosstrees on a ship, supporting the jackstaff and often used as a lookout. Also: either of a pair of crosstrees at the head of the topgallant mast, supporting the royal and skysail masts.

Origin

Mid 19th century; earliest use found in Boston Pearl. From Jack + cross-tree.

Pronunciation

jackcrosstree

/ˌdʒakˈkrɒstriː/