The threshold was a log or length of wood placed in the entranceway of the peasant's hut  or accommodation provided by the landlord. It contained within the hut the layers of thresh i.e. waste from harvesting corn or wheat. It built up in layers.
The newly wed husband would carry his wife across the thresh hold (threshold) to a comfortable  area inside the dwelling - hence they  would cross the threshold.
by Philip Hoesli July 6, 2006
Get the cross the thresholdmug.