1 Syllableisn't, peon
Now you can make multisyllabic rhymes in the new rhymit app :)
2 Syllablesaeon, augean, beacon, beadsman, chaldean, cheapen, chieftain, clearance, daemon, deacon, decent, deepen, deepend, demon, demond, didn't, eland, eonard, even, fecund, free-man, freeman, grievance, heathen, isn'tis, korean, legion, lesion, lethean, needments, orphean, paean, paeon, pandean, peonid, piquant, preachment, preachments, reason, recent, regent, regent's, regents, region, riband, seaman, season, seedsman, sejant, semen, sequence, sequent, sheepman, steepen, stridden, thriven, treason, treatment, treatment's, treatments, weaken, weazen
3 Syllablesaccretion, adherence, adherent, adherents, adhesion, agreement, agreement's, agreements, appearance, archaean, archdeacon, beaconed, bedesman, cheapened, coherence, coherent, cohesion, completion, concretion, cosecant, cyclopean, deaconhod, decentnes, deepened, defeasance, deletion, deletiontfd, demonoid, empyrean, european, evened, evenold, evenrud, excretion, genevan, grecian, heathenhod, helvetian, herculean, impeachment, impeachments, indecent, infrequent, inherence, inherent, inhesion, jacobean, laodicean, legioned, malfeasance, misfeasance, mohican, piquantnes, prescription, reasoned, recentnes, recentres, regentes, regentess, regioned, repletion, seasoned, secretion, sequenced, sisyphean, spelaean, steepened, trihedron, unbeaten, uneven, unreason, venetian, weakened, weazened
4 Syllablesachievement, achievements, allegiance, appearanced, artesian, bodleian, cartesian, collegian, decentius, defeasanced, disagreement, disagreement's, disagreements, disappearance, epicurean, glaswegian, hexahedron, hyperborean, incoherence, incoherent, indecentnes, interference, milesian, misdemeanant, misdemeanants, mycenaean, norwegian, octahedron, pentahedron, perseverance, plebeian, policeman, polyhedron, pythagorean, tetrahedron, unreasoned, unseasoned, venetianed, vicegerent
5 Syllablesdodecahedron, elizabethan, incoherentnes, indonesian, polynesian