Weight. See: Cart-load. About 19½ hundredweight, dependent upon material. Also, six sacks where each sack is five fotmal. Also, "lots of," "a large quantity," "a huge amount," etc.
A wain is a type of horse-drawn, load-carrying vehicle, used for agricultural purposes rather than transporting people, for example a haywain. It normally has four wheels but the term has now acquired slightly poetical connotations so is not always used with technical correctness. However, a two-wheeled 'haywain' would be a hay cart.