Wiktionary
mainchain
n. (context nautical mostly plural English) A chain securing the lower shrouds of the mainmast.
Usage examples of "mainchain".
He wanted to shut his eyes as the schooner hit the jibboom but was even too frightened for that Suddenly the man on the jibboom jerks as though shot - now the thin snake of the heaving line is darting towards the schooner's mainchains.