The Collaborative International Dictionary of English v.0.48:

Ratlines \Rat"lines\, Ratlins \Rat"lins\ (r[a^]t"l[i^]nz), n.
   pl. [Of uncertain origin.] (Naut.)
   The small transverse ropes attached to the shrouds and
   forming the steps of a rope ladder. [Written also ratlings,
   and rattlings.] --Totten.
   [1913 Webster]