Thickness rhymes with:
quickness, sickness, blackness, bleakness, darkness, frankness, harkness, hawkiness, homesickness, likeness, preakness, quickness, sickness, slackness, uniqueness, weakness
Thickness sounds like:
tacking, tacoma's, taganka, tagging, takanashi, taking, takings, tasking, taxiing, taxing, teaching, teachings, teasing, technic, technics, technique, techniques, technomic, tecumseh, texan's, texans, thesing, thickening, thickens, thyssen's, ticinus, ticking, tokens, tokunaga, toscanini's, tossing, touching, toughening, toughens, toughness, toxins, tsang, tseng, tsiang, tsongas, tsongas', tsongas's, tucking, tudjman's, tugging, tusing, tussing, tweaking, twitching, tycoon's, tycoons, tyson's, tysons