A hairshirt (or Cilice) is an undergarment worn to induce discomfort, and thereby lead the wearer to spiritual purity, salvation, or atonement for sin.
As penance for his sins, and echoing his own punishment of the Saxons after the First Battle of Langensalza, he dramatically wore a hairshirt and stood in the snow barefoot in the middle of winter in what has become known as the Walk to Canossa.