late 14c., from barley + corn (n.1). Perhaps to distinguish the barley plant or the grain from its products. In Britain and U.S., the grain is used mainly to prepare liquor, hence personification as John Barleycorn (1620) in popular ballad, and many now-obsolete figures of speech, such as to wear a barley cap (16c.) "to be drunk."
1. A barleycorn is better than a diamond to a cock.