Frivolous rhymes with:
marvelous, acropolis, aeschelus, anomalous, boulalas, calculus, carolus, credulous, daedalus, deangelis, fabulous, garrulous, heliopolis, hydroencephalus, incredulous, libelous, los-angeles, maquilas, marvelous, mcnicholas, megalopolis, merciless, meticulous, metropolis, miraculous, nautilus, nebulous, necropolis, nicholas, nicholas', nicolas, oscalus, oxalis, papadopoulos, pendulous, perilous, persepolis, populace, populous, querulous, ridiculous, romulus, scandalous, scrupulous, scurrilous, segalas, stefanopolis, stephanopoulos, stephanopoulos', stimulus, tantalus, tremulous, unscrupulous
Frivolous sounds like:
fireballs, fireflies, fireplace, fireplaces