1) The amount it took to fill a skep.
This was a rare term, possibly because for much of its history the skep had a precise meaning. The OED has one example in Durham as early as c.1570 where the materials in the skep were sand and clay, but no others until the nineteenth century. In York, in 1579, it was agreed that anybody who deposited dung or filth at Skeldergate postern should be fined 12d for every skepfull or burden. Elsewhere, of course, a ‘burden’ was deemed to be the amount that a man might carry, as opposed to the ‘horse load’ or ‘cart load’.