trĭnundĭnus — Lewis & Short
trĭnundĭnus, a, um, adj.tres nundinae,
I of or belonging to three weekly market-days (very rare):
trinundino die,after seventeen days, Macr. S. 1, 16, 34; v. trinus, II. B., and nundinus, III. — Hence, adv.: trĭnundĭnō, = trinundino die, Macr. S. 2, 13, 7.