Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. ultĭmō, adv., v. ultimus, under ulter, II. B. 1. fin.
2. ultĭmo, āre, v. n. [ultimus], to come to an end, be at the last: cum ultimarent tempora patriae, Tert. Pall. 1 fin.