Underside rhymes with:
fireside, harborside, ketcherside, riverside, silverside, alcide, backside, bayside, beachside, bedside, blindside, broadside, brookside, burnside, cambridgeside, cityside, countryside, curbside, dioxide, dockside, downside, exide, fireside, fratricide, fungicide, garside, gartside, genocide, germicide, glucoside, glycoside, graveside, harborside, herbicide, hillside, homicide, infanticide, insecticide, ironside, ketcherside, lakeside, mountainside, oceanside, oxide, parkside, peroxide, pesticide, poolside, prophesied, ringside, riverside, roadside, seaside, silverside, southside, suicide, sunnyside, superoxide, wayside, whiteside, woodside
Underside sounds like:
unaddressed, unauthorized, undercoat, undercooked, undercut, undersized, understate, understated, understood, understudy, underused, underweight, underweighted, undressed, uninterested