Definition of main-topgallant in US English:

main-topgallant

noun

Nautical
  • A topgallant situated just above the head of the maintopmast. Chiefly in compounds: "main-topgallant-crosstree", "main-topgallant-masthead", "main-topgallant-sail", "main-topgallant-yard", etc. "main-topgallant-mast"noun the mast above the maintopmast.

Origin

Late 16th century. From main + topgallant.