A bulwark, a rampart. Also figurative : something which affords defence or protection.
Origin
Late 18th century; earliest use found in James Boswell (1740–1795), lawyer, diarist, and biographer of Samuel Johnson. From classical Latin prōpugnāculum bulwark, rampart, defence, protection, defence in argument from prōpugnāre + -culum. Compare Spanish propugnáculo, Portuguese propugnáculo, and Italian propugnacolo.