Generally speaking, a "cockbag" is a reference to a plastic recepticle other than a condom or grocery bag used to prevent unwanted pregnancies.
May refer to a bag-like structure used to house male genitals.
Can also be used as a derogatory term to psychologically harm another person.
1) When I was engaging in sexual intercourse with your mother, my cockbag busted and my seed spilled into her bat cave.
2) You are a cockbag, you bitch ass nigga.