Thinnest rhymes with:
abortionist, africanist, agonist, anticommunist, balloonist, brownest, collaborationist, colonist, constitutionist, constructionist, earnest, expansionist, extortionist, funnest, greenest, humanist, hygienist, internist, interventionist, leninist, misogynist, noncommunist, nutritionist, opportunist, percussionist, peronist, preservationist, protectionist, receptionist, rejectionist, religionist, reservationist, revisionist, revolutionist, satanist, saxophonist, secessionist, segregationist, stalinist, tannest, trombonist, zionist
Thinnest sounds like:
tanked, tannest, teenaged, tenacity, thanked, tinged, tiniest, tomasetti, tomassetti, tomcat, tongued, toniest, tonight, tungate, tunicate, twinjet