Wiktionary
main-truck
n. (context nautical English) On a wooden mast, a circular disc (or sometimes a rectangle) of wood near or at the top of the mast, usually with holes or sheaves to reeve signal halyards; also a temporary or emergency place for a lookout. "Main" refers to the mainmast, whereas a ''truck'' on another mast may be called (on the mizzenmast, for example) "mizzen-truck".
Usage examples of "main-truck".
A sky-hawk that tauntingly had followed the main-truck downwards from its natural home among the stars, pecking at the flag, and incommoding Tashtego there.