Slackness rhymes with:
blackness, blackness, blackness, bleakness, darkness, frankness, harkness, hawkiness, homesickness, likeness, preakness, quickness, sickness, thickness, uniqueness, weakness
Slackness sounds like:
salesman's, salesmen's, silicones, siliconix, slackening, slackens, slashing, slicing, slocan's, slogans, slogging, sloshing, slouching, slugging, sluggishness, sluicing, socializing, squelching, sulking