ratline n.

"thin rope," especially, on sailing ships, one of a series of small ropes or lines which form the steps of ladders for going aloft, late 15c., ratling, also ratlin, a word of obscure etymology. Compare Dutch weeflijn, German Webeleine "web line." The spelling ratline is attested from 1773, perhaps by folk etymology influence of rat (n.) + line (n.), "a seamen's jocular name, as if forming ladders for the rats to climb by" [Century Dictionary].