Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. sĕnĕcĭo, ōnis, m. [senex], an old man, Afran. ap. Prisc. p. 618 P.
2. Sĕnĕcĭo, ōnis, m. [senex], a Roman surname, Tac. Agr. 2; 45; id. A. 13, 12; 15, 50; 15, 56 sq.; Plin. Ep. 3, 11, 3.
3. sĕnĕcĭo, ōnis, m., a plant, called also erigeron, groundsel, Plin. 25, 13, 106, § 167; App. Herb. 75.