The Hebrew word for mark ('Oth, אות) could mean a sign, an omen, a warning, or a remembrance.[11] The mark of Cain is God's promise to offer Cain divine protection from premature death with the stated purpose of preventing anyone from killing him. It is not known what the mark was, but it is assumed that the mark was visible.[12] Some have speculated that the mark was a Hebrew or Sumerian letter placed on either the face or the arm.[13] The Septuagint translates the mark as a "sign." Thus, it is speculated that the mark served as a sign to others not to commit the same offense.[14][15]
https://en.wikipedia.org/wiki/Curse_and_mark_of_Cain