In common use it generally refers to a generic non-measurement of any kind in a similar vein to “widgets”, however it is actually a speed.
The barleycorn was used as a measurement of length in early Anglo-Saxon times, now, generally accepted as one third of an inch.
Therefore your average person walks at approximately 127,733,760 barleycorns per fortnight.
2 miles per hour
== 10,560 feet per hour
== 126,720 inches per hour
== 380,160 barleycorns per hour
== 9,123,840 barleycorns per day
== 63,866,880 barleycorns per week
== 127,733,760 barleycorns per fortnight
And, to answer the question
A barleycorn per fortnight is 3.91439 x 10-9 mph
Interestingly, a poppy seed is a quarter of a barleycorn.
