Stiffening rhymes with:
deafening, orphaning, siphoning, softening, toughening, abandoning, airconditioning, apportioning, auctioning, auditioning, awakening, battening, beckoning, blackening, bludgeoning, boening, brightening, broadening, burdening, burgeoning, captaining, captioning, cautioning, championing, cheapening, chickening, christening, clendening, commissioning, conditioning, corpening, cushioning, dampening, darkening, deadening, deafening, decommissioning, deepening, dekoning, determining, disciplining, disheartening, disillusioning, enlightening, enlivening, envisioning, fashioning, fastening, fattening, flattening, freshening, frightening, functioning, gardening, glendening, glistening, groening, happening, hardening, harmening, hastening, heartening, heightening, imagining, imprisoning, jettisoning, kroening, leavening, lengthening, lessening, lightening, likening, listening, loosening, maddening, malfunctioning, margining, mentioning, moening, motioning, opening, optioning, orphaning, pardoning, partitioning, petitioning, poisoning, positioning, provisioning, questioning, quickening, rationing, reasoning, reawakening, reckoning, reconditioning, reexamining, reopening, repositioning, ripening, saddening, sanctioning, schoening, seasoning, sectioning, sharpening, shortening, sickening, siphoning, slackening, smartening, softening, stationing, straightening, strengthening, summoning, sweetening, terpening, thickening, threatening, tightening, toughening, transitioning, tregoning, unquestioning, vacationing, weakening, widening, wilkening, worsening
Stiffening sounds like:
skydiving, stabbing, stabbings, staffing, staving, stebbins, steepness, stefani's, stefaniak, stefanic, stefanich, stefanick, stefanik, stefanko, stefanowicz, stefanski, steffanci, steffens, stepanek, stepanian's, stepanski, stephans, stephen's, stephens, stephens's, stepping, steven's, stevens, stevens', stevens's, stiffens, stiffness, stooping, stopping, stubbins, stuffing, suggestiveness