An alcoholic drink (typically malt or fortified liquor) such as Mad Dog 20/20, Wild Irish Rose, Colt 45, Steel Reserve, Milwaulkees Best, King Cobra, Olde English, etc...
This drink when purchased comes in a single brown carrying bag (sometimes called a "skirt") that is used to insulate it from the hand while it is consumed. If the drink is consumed while in a motor vehicle, it may also be referred to as a "traveler".
The drinks title is a derivative of the "Tom Collins", as to make it sound classier when it is described to others.
The Tom Cobbins is a favorite of lower income mavros, often Taco Bell workers. The drink is provides a strong, but reliable cheap buzz.
"I'd like to get a "Rosey in a Skirt'."
"I need to run over to the BP and grab me a few Tom Cobbins before the boss gets back".
"Hey whats in the brown bag?
I got me a Blue Raspberry Bling MD 20/20 Tom Cobbins"