Achuckwagon used to be a wagon carrying cooking facilities and food for serving men working outdoors, as at a ranch or lumber camp. This is what a chuckwagon looked like:

What exactly is a chuckwagon?
It is a covered wagon with an upright chuckbox (a kind of kitchen cabinet) at the rear. The chuckwagon originally was an essential vehicle on the long cattle drives for it not only carried food for the cowboys but also their bedrolls and other equipment.The cook was usually the second highest paid of the trail crew, the trail boss being number one.
The traditional design of the chuckwagon was attributed to cattle king Charles Goodnight, who in 1866 adapted an army wagon for the purpose. The chuckbox was fitted with compartments to hold provisions and utensils, and its tailgate door dropped down to form a table on which the cook could work. The wagon carried a water barrel, flour box, coffee grinder, tool box and all essential implements; a leather sling under the wagon carried wood and dried buffalo 'chips' (dung) for the fire, which was made in a pit to avoid setting the prairie alight.
The chuckwagon was the focal point of a trail or round-up camp and here the cowboys would eat, tell stories and sing. A favorite dish was 'son-of-a-bitch' stew of which every cook had his own version; in fact almost anything went into it. There was also plenty of beef available and it seems that cowboys liked their steaks well done and their coffee strong and black. A great delicacy was the broiled testicles of a steer, which the cowboys called 'prarie oyster'.