A slick, in Army slang, is a cargo vehicle, whether it has wheels or a rotor. The name probably derives from the fact that most trucks and quite a few of the Army's helicopters are carrying stuff inside that gets in the way of passengers and cargo. Most units kept a couple of slicks around for the kind of thing suburban mothers use SUVs for: picking up food, carrying the kids to the bathhouse, and running various errands for which the unit's normal vehicles, whether they be intercept vans or jamming jeeps or tracks, would be utterly unsuitable.
The most common slick during my time in the 331st ASA Company and the 523rd ASA Company (1980-85) was the gasoline-fueled M880 Dodge 5/4 ton pickup truck, widely believed to be the bastard child of Congressional pork and Chrysler's dire need for money. As the 880s got older and the Army got tired of having to lug two kinds of fuel all over the place, these were replaced by M1008 Chevy diesels and mil-spec versions of the Blazer SUV. These slightly modified civilian vehicles worked fairly well in a training environment, especially in Western Europe, which had a zillion roads and not too much really rough country. Unfortunately, when the Army had to take these trucks to other parts of the world, they proved to be not well suited for extended off-road/no-road use and wound up being replaced by the HMMWVs they were supposed to complement.
"Slick" does not refer to the M151 jeep or to any truck bigger than a deuce, just as one does not have a "slick" Chinook.
The_Custodian has wisely suggested that I include the following:
In my part of Vietnam, they were just as often called Slicks as Hueys. The origin of the term may be two-fold. Before the AH-1 Cobra appeared in 1967, UH-1s had been fitted with side-mounted guns—improvised gunships. Those without guns were "slick-sided" or just Slicks. The Marine origin of the term may refer to the UH-1's ability to fly without the internal seat frame arrangement, just a "slick deck." Either way, the UH-1 made its mark.