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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.