accompany v.

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.

c. 1300, "one who accompanies or associates with another," from Old French compagnon "fellow, mate, friend, partner" (12c.), from Late Latin companionem (nominative companio), literally "bread fellow, messmate," from Latin com "with, together" (see com-) + panis "bread," from PIE root *pa- "to feed."

The Late Latin word is found first in the 6c. Frankish Lex Salica, and probably it is a translation of some Germanic word (compare Gothic gahlaiba "messmate," from hlaib "loaf of bread"). It replaced Old English gefera "traveling companion," from faran "go, fare."

The meaning "A person who lives with another in need of society, and who, though receiving remuneration, is treated rather as a friend and equal than as an inferior or servant" [OED] is from 1766.