Taking a shit. "Cosbies" refers to the Cosby Show, in which Bill Cosby had approximately fifteen kids. You can replace "Cosby" with the name of one of the kids from the show, i.e. - "Dropping Off A Theo" to indicate a large dump, or "Dropping Off a Rudy" to indicate a smaller turd.
Goddamn man, pull the car over, I need to drop off some Cosbies.