Why is the touchline so named? What is the origin of “touch” and “in touch”?

Answer (September 5, 2014):
“Touch” is any area outside the boundaries of the field, particularly the lines that run between the corners across the halfway line to the corner at the far end of the field. It is the area in which the ball may be handled legally by players, i.e., “touched.” Once the whole of the ball has crossed the whole of the boundary line, it is “in touch.”