Backup rhymes with:
breakup, buttercup, checkup, eggcup, hookup, linkup, lockup, makeup, markup, pickup, shakeup, stalcup, stallcup, teacup, wakeup, walkup, wisecup
Backup sounds like:
bagby, bagpipe, bagsby, bake-off, basayev, bigbee, bigbie, bigby, bigsby, bisbee, bischof, bischoff, bishoff, bishop, biskup, bixby, bizub, bookshop, bovespa, buckbee, bucoba, bugaboo, bugbee, bukavu, busbee, busbey, busboy, busby, bushby, buzbee, buzby