Ultimatum rhymes with:
mentholatum, tatum, verbatim, accustom, ageratum, antemortem, arboretum, argosystem, atom, autumn, bantam, biosystem, bottom, cablesystem, centum, chatham, cheatham, cheatum, chittum, cottam, cottom, custom, datum, dictum, eastham, ecosystem, frantom, haltom, higginbottom, infinitum, item, kortum, lineitem, mentholatum, momentum, mortem, ottum, partum, phantom, quantum, rectum, sanctum, sense-datum, statum, stratum, subsystem, symptom, system, tatom, tatum, techsystem, telesystem, totem, ultrasystem, verbatim, victim, wintom