Cc used to stand for 'carbon copy', in the days that carbon paper was used between the original and the copy. At the bottom of the page, "Cc" was followed by the names of people other than the original recipient who would get copies of the letter. It is still used today in emails: and an additional "Bcc" to indicate others to receive the email without their addresses being exposed to the others.

"C/o" is what I think you have in mind. It is used on an envelope to say "in care of". I send a letter to my friend who is staying with another person for a while: I address the letter John Jones - c/o Susan Smith - and then her address.

Recently, c/o has also been used by students to refer to themselves as the c/o (class of) 2008.
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