abuilding (adj.)

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.

prefix or inseparable particle, a conglomerate of various Germanic and Latin elements.

In words derived from Old English, it commonly represents Old English an "on, in, into" (see on (prep.)), as in alive, above, asleep, aback, abroad, afoot, ashore, ahead, abed, aside, obsolete arank "in rank and file," athree (adv.) "into three parts," etc. In this use it forms adjectives and adverbs from nouns, with the notion "in, at; engaged in," and is identical to a (2).

It also can represent Middle English of (prep.) "off, from," as in anew, afresh, akin, abreast. Or it can be a reduced form of the Old English past participle prefix ge-, as in aware.

Or it can be the Old English intensive a-, originally ar- (cognate with German er- and probably implying originally "motion away from"), as in abide, arise, awake, ashamed, marking a verb as momentary, a single event. Such words sometimes were refashioned in early modern English as though the prefix were Latin (accursed, allay, affright).

In words from Romanic languages, often it represents reduced forms of Latin ad "to, toward; for" (see ad-), or ab "from, away, off" (see ab-); both of which by about 7c. had been reduced to a in the ancestor of Old French. In a few cases it represents Latin ex.

[I]t naturally happened that all these a- prefixes were at length confusedly lumped together in idea, and the resultant a- looked upon as vaguely intensive, rhetorical, euphonic, or even archaic, and wholly otiose. [OED, 1989]