This adverb describes a heavy, difficult, or sluggish manner of action, derived from the verb meaning to weigh down or burden. In both New Testament occurrences, it modifies hearing: people hear 'heavily' or 'with dull ears,' unable to truly understand because their sensory and mental faculties have become sluggish. Matthew 13:15 and Acts 28:27 both quote Isaiah 6:9-10, where God describes a people whose ears have become dull and heavy. This isn't physical deafness but a willful, hardened inability to truly hear and respond to God's word. The word suggests weight, burden, and the difficulty of perception when hearts resist truth.