Stiffen rhymes with:
giffen, griffen, griffon, kniffen, sniffen, acetaminophen, altenhofen, carstarphen, dalfen, deafen, dextrathoraphan, dolphin, draffen, entoffen, farfen, geffen, giffen, griffen, griffon, hafen, hanifen, hyphen, ibuprofen, kniffen, lafon, ludwigshafen, mcguffin, mcmuffin, morphin, muffin, offen, often, orphan, paraffin, siphon, sniffen, soften, steffan, steffen, stephan, sutphen, syphon, tamoxifen, toughen, xenophon
Stiffen sounds like:
sitiveni, stabenow, staubin, stavinoha, stefan, stefani, stefanie, stefano, steffan, steffen, stepan, stepanian, stephan, stephana, stephane, stephani, stephania, stephanie, stephany, stephen, stepien, stepney, steppin', steuben, stevana, steven, stevena, sutfin, sutphen, sutphin