aloud (adv.)

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," etc., forming adjectives and adverbs from nouns, with the notion "in, at; engaged in." In this use it 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 are examples).

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]

Middle English, from Old English hlud "noisy; making or emitting noise" (of voices, musical instruments, etc.), from Proto-Germanic *hludaz "heard" (source also of Old Frisian and Old Saxon hlud, Middle Dutch luut, Dutch luid, Old High German hlut, German laut "loud"), from PIE *klutos- (source also of Sanskrit srutah, Greek klytos "heard of, celebrated," Latin inclutus "renowned, famous," Armenian lu "known," Irish cloth "noble, brave," Welsh clod "praise, fame"), suffixed form of root *kleu- "to hear."

Of places, "noisy," from 1590s. Application to colors, garments, etc. ("flashy, showy") is by 1849. Also used colloquially of notably strong or bad smells. Paired with clear (adj.) at least since c. 1650.