Callously rhymes with:
aimlessly, breathlessly, carelessly, ceaselessly, effortlessly, endlessly, fabulously, flawlessly, frivolously, fruitlessly, harmlessly, helplessly, hopelessly, jealously, listlessly, marvelously, mercilessly, meticulously, mindlessly, miraculously, needlessly, overzealously, painlessly, perilously, recklessly, relentlessly, restlessly, ridiculously, ruthlessly, scrupulously, seamlessly, shamelessly, tirelessly, tremulously, zealously
Callously sounds like:
ceaselessly, chalkley, classical, classically, clausell, cleckley, closely, coleslaw, collegial, colloquial, colossal, colossally, cyclical