Flesh — the physical body, human nature in its weakness, or the sinful disposition opposed to the Spirit. Sarx has a wide range in the NT: neutral (physical body), positive (the Word became flesh), and negative (the works of the flesh). Paul uses it especially for humanity's fallen inclination.
'The Word became flesh' (John 1:14) — sarx at its most dignified, God entering human physicality. But Paul warns: 'Those who live according to the flesh set their minds on things of the flesh' (Romans 8:5). The Spirit-flesh conflict (Galatians 5:17) is not body versus soul but God's way versus humanity's fallen way. The flesh is not the body itself but the whole person oriented away from God.