Origin and history of agglomeration

late 14c., "soil of the earth; cultivated land;" also "a piece of land forming part of a clergyman's benefice," from Old French glebe, from Latin gleba, glaeba "clod, lump of earth," possibly from a PIE *glem- or *glom-, which might mean "contain, embrace" or "ball," or might be two different roots. Possible cognates include Old English clamm "a tie, fetter;" Old High German klamma "trap, gorge;" Old Irish glomar "gag, curb;" Latin globus "sphere," gleba, glaeba "clod, lump of earth;" Old English clyppan "to embrace;" Lithuanian glėbys "armful," globti "to embrace, support."

word-forming element expressing direction toward or in addition to, from Latin ad "to, toward" in space or time; "with regard to, in relation to," as a prefix, sometimes merely emphatic, from PIE root *ad- "to, near, at."

Simplified to a- before sc-, sp- and st-; modified to ac- before many consonants and then re-spelled af-, ag-, al-, etc., in conformity with the following consonant (as in affection, aggression). Also compare ap- (1).

In Old French, reduced to a- in all cases (an evolution already underway in Merovingian Latin), but French refashioned its written forms on the Latin model in 14c., and English did likewise 15c. in words it had picked up from Old French. In many cases pronunciation followed the shift.

Over-correction at the end of the Middle Ages in French and then English "restored" the -d- or a doubled consonant to some words that never had it (accursed, afford). The process went further in England than in France (where the vernacular sometimes resisted the pedantic), resulting in English adjourn, advance, address, advertisement (Modern French ajourner, avancer, adresser, avertissement). In modern word-formation sometimes ad- and ab- are regarded as opposites, but this was not in classical Latin.