barn n.

"covered building for the storage of farm produce," Middle English bern, bærn, from Old English bereærn "barn," literally "barley house," from bere "barley" (see barley) + aern "house; place for storing," metathesized from *rann, *rasn (source also of Old Norse rann "large house," Gothic razn "house," Old English rest "resting place").

For the formation and the second element, compare saltern "a salt-works," from Old English sealtærn "saltworks;" Old English horsern "stable." Latin cellarium was glossed by Old English hordern, and dormitorium was slæpern.

In Anglo-Saxon England, barley was a primary grain crop:

Barley was not always the only crop grown as the data recovered at Bishopstone might suggest but it is always the most commonly represented, followed by wheat and then rye and oats. [C.J. Arnold, "An Archaeology of the Early Anglo-Saxon Kingdoms," 1988, p.36]

Another word for "barn" in Old English was beretun, "barley enclosure" (with tun "enclosure, house"), which accounts for the many Barton place names on the English map and the common surname.

It was applied from early 18c. to any large, barn-like building. Barn door has been used figuratively for "broad target" since 1670s and "great size" since 1540s. Barn-owl is attested by 1670s. Barn-raising "a collective effort by neighbors or community members to erect the frame of a barn for one of them, accompanied by a social gathering" is attested by 1849.