ultimatum n.

"final demand," 1731, from Modern Latin, from Medieval Latin ultimatum "a final statement," noun use of Latin adjective ultimatum "last possible, final," neuter of ultimatus (see ultimate). The Latin plural ultimata was used by the Romans as a noun, "what is farthest or most remote; the last, the end." In slang c. 1820s, ultimatum was used for "the buttocks."