The Greek verb astheneō means to be weak, sick, without strength, or infirm. It encompasses both physical illness and spiritual/moral weakness. The root astheneia (weakness/infirmity) is pervasive in Paul's theology.
Paul's treatment of weakness (astheneia/astheneō) is one of the most counter-cultural aspects of New Testament theology. In his own life, Paul reports that when he boasted in his weaknesses, Christ's power rested on him — 'for when I am weak, then I am strong' (2 Corinthians 12:10). The 'weak in faith' (Romans 14-15) must be received, not judged. Christ's crucifixion is described as done 'in weakness' yet is the power of God (2 Corinthians 13:4). Jesus himself 'took up our infirmities' (Matthew 8:17, citing Isaiah 53:4). The theology of weakness inverts human values: God's strength is perfected in, not despite, human frailty.