A (probably) derogatory term used by 'truckers' (i.e. operators of 18-wheel tractor-trailers) to describe Station Wagons. I don't think it would work now-a-days, see as how some station wagons, like the Subaru Outback, are actually kind of cool; probably applies to the old k-car wagon varieties. Think faux wood paneling.

The term "Pregnant Rollerskate" was CB and/or trucker slang for a Volkswagen Beetle, referring to both the shape (i.e., like a pregnant woman's belly) and the size (rather small) of the car.

The classic Beetle (aka the VW Bug) certainly has the right shape. The curve of the passenenger compartment would be the belly, the headlights would be where the legs connect to the hips, and I'll let your mind go from there.

"Break One-Nine for the rolling parking lot with the pregnant roller skates, ya got yer ears on?"