outbuilding n.

in Old English a common prefix with nouns, adjectives, adverbs, and verbs, "out, outward, outer; forth, away," from out (adv.). The use was even more common in Middle English, and also with the senses "outer, outside, on the outside, from without, external, externally; apart; greatly, extremely; completely, thoroughly, to completion." Other senses of out that extended into the use as a prefix include "beyond the surface or limits; to the utmost degree; to an explicit resolution."

In composition out has either its ordinary adverbial sense, as in outcast, outcome, outlook, etc., or a prepositional force, as in outdoors, or forms transitive verbs denoting a going beyond or surpassing of the object of the verb, in doing the act expressed by the word to which it is prefixed, as in outrun, outshine, outvenom, etc. In the last use especially out may be used with almost any noun or verb. [Century Dictionary]

c. 1300, "a structure;" late 14c., "act or process of constructing;" verbal noun from build (v.). Building-block is attested from 1846 as "one of a set of children's play blocks;" 1849 as "temporary support on which a ship's keel rests while the ship is being constructed;" 1856 as "cinder-block, concrete block, artificial stone block used in building construction." The figurative sense of "basic unit from which something is constructed" is by 1955.