1. Short for Insulation Displacement Contact or Insulation Displacement Connector, IDC is a connector found at the end of many cables. For example, an IDE disk drive or floppy disk drive cables have IDCs at the ends of the ribbon cable. In the picture below is an example of an Insulated Displacement Connectors on a IDE cable.
2. Shorthand for I don't care, IDC is sometimes used in chat and other text-based communications as letting others know you don't care about what is being discussed.