tertĭus-dĕcĭmus — Lewis & Short
tertĭus-dĕcĭmus (-dĕcŭmus), a, um,
I num. ord. adj., the thirteenth:
dies,Cels. 3, 4 med.:
mense,Col. 6, 36, 2; Plin. 8, 44, 69, § 171:
tertiadecuma legio,Tac. A. 1, 37.