noun

the quality or condition of being hard.

people complained about the hardness of the chairs

firmness,solidity,stiffness,rigidity,denseness,inflexibility,inelasticity,resistance,hard-heartedness,heartlessness,cold-heartedness,lack of compassion,lack of feeling,lack of sentiment,callousness,unkindness,flintiness,steeliness,harshness,severity,strictness,sternness,toughness