Grownup rhymes with:
burnup, canup, cleanup, greenup, lineup, runup, tuneup, backup, blowup, breakup, buildup, burnup, buttercup, canup, checkup, cleanup, close-up, coverup, crudup, eggcup, followup, greenup, hangup, holdup, hookup, jessup, letup, lineup, linkup, lockup, makeup, markup, northup, pickup, pileup, printup, push-up, pushup, roundup, runup, setup, shakeup, speedup, stalcup, stallcup, standup, start-up, startup, teacup, tuneup, wakeup, walkup, warmup, windup, wisecup
Grownup sounds like:
granoff, greenup, grump, grumpy