This expression is an idiom. As such it could have a meaning that extends beyond the literal, word-for-word translation.
Literal meaning: ‘ot’ is either a sign, a mark, or a letter (character). Ka•yin is the name of the first Biblical murderer, Cain. Ot Ka•yin then is the mark that God placed on Cain’s forehead.
“And the Lord set a mark upon Cain, lest any finding him should kill him”
The intention of that mark was twofold: 1. to mark Cain as a murderer and hereby warn others from him; but this reason is not even mentioned in the Bible. Cain’s punishment was to be cursed by the earth that would not yield her strength to him anymore and to become a wanderer on the earth. The reason for the ‘ot’ was actually to give sanctuary to Cain, the murderer, despite his action.
This is the very first protective act given by God that sets the limits of harm that one person can inflict on another. Many other sanctuaries are introduced in the Bible as Law and have the same protective intention, such as, for example, the bankruptcy laws, she•mi•tat cho•vot (dropping of the debts) that limits the harm a lender can cause to a debtor.
Extended meaning: The idiom ‘ot Ka•yin’ can be used to point out a person that is known for doing very bad actions in the past. That ‘ot Ka•yin’ ‘reputation’ follows the person anywhere he or she goes. In today’s practice, courts allow (and even require) attaching an ‘ot Ka•yin’ to certain sex offenders by making their names and addresses known to the public