accustom 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. 1200, custume, "habitual practice," either of an individual or a nation or community, from Old French costume "custom, habit, practice; clothes, dress" (12c., Modern French coutume), from Vulgar Latin *consuetumen, from Latin consuetudinem (nominative consuetudo) "habit, usage, way, practice, tradition, familiarity," from consuetus, past participle of consuescere "accustom," from com-, intensive prefix (see com-), + suescere "become used to, accustom oneself," related to sui, genitive of suus "oneself," from PIE *swe- "oneself" (see idiom).

Custom implies continued volition, the choice to keep doing what one has done; as compared with manner and fashion, it implies a good deal of permanence. [Century Dictionary]

A doublet of costume. An Old English word for it was þeaw. Meaning "the practice of buying goods at some particular place" is from 1590s. Sense of a "regular" toll or tax on goods is early 14c. The native word here is toll (n.).

Custom-house "government office at a point of import and export for the collection of customs" is from late 15c. Customs "area at a seaport, airport, etc., where baggage is examined" is by 1921.

Old customs! Oh! I love the sound,

  However simple they may be:

Whate'er with time has sanction found,

  Is welcome, and is dear to me.

Pride grows above simplicity,

  And spurns it from her haughty mind,

And soon the poet's song will be

  The only refuge they can find.

[from "December," John Clare, 1827]