2 Syllables: SU
biggest, bigness, citrus, fitness, fittest, hideous, idiots, igneous, mixes, Nicholas, nitrous, piteous, prettiest, quibus, quickness, sickness, strictest, strictness, thickness, vitreous, witness
3 Syllables: USU
amphibious, assiduous, enigmas, eyewitness, fastidious, insidious, invidious, Mauritius, perfidious, unfitness
Found 31 words across 2 groupings. Skipped 1 excess word of a similar stem.