שמואל א יד:יד פירוש: רש״י

Half a furrow, of a pair [of oxen plowing] in the field. Within the area of a land measure [equal to] half the amount of a furrow of the plowing of a pair of oxen plowing a field.

Furrow. That is the ridge which is called 'reie' in O.F. This was a very mighty deed, because they were near each other and ready to help each other.