Flabbergast rhymes with:
pendergast, prendergast, pendergast, prendergast, amcast, belfast, broadcast, bypassed, chloroplast, colorfast, comcast, downcast, enthusiast, forecast, holdfast, iconoclast, lightfast, newscast, outcast, outclassed, outlast, overcast, pendergast, polycast, prendergast, qualcast, rebroadcast, sandblast, simulcast, steadfast, superfast, telecast, tillinghast, triplecast, typecast
Flabbergast sounds like:
flabbergasted, flavorist, fulbright, fullbright