c/oabbr. care ofAmerican Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.c/o abbreviation for 1. care of 2. (Accounting & Book-keeping) accounting carried over Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014CO 1. Colorado. 2. Commanding Officer. 3. conscientious objector. Co Chem. Symbol. cobalt. co- var. of com- before a vowel, h, and gn: coalesce; cohere; cognate. The prefix co-, with the sense ?joint, jointly,? now forms new words from bases beginning with any sound (cochair; cogeneration; costar; coworker), sometimes with the derived sense ?auxiliary? (coenzyme; copilot), and, in mathematics and astronomy, with the sense ?complement? (codeclination). Co. or co., 1. Company. 2. County. C/O certificate of origin. C/o or c/o, care of. C.O. 1. Commanding Officer. 2. conscientious objector. c.o. 1. care of. 2. carried over. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.