Douglas Harper's Etymology Dictionary
c/o
addressing abbreviation for care of; "common" by 1889.
Wiktionary
c/o
abbr. 1 care of 2 [certificate of origin] a document used for shipping purposes. Describes country of origin of a given product. alt. 1 care of 2 [certificate of origin] a document used for shipping purposes. Describes country of origin of a given product.