Origin and history of adhesion

mid-15c., "steady attachment of the mind or feelings to a person, cause, belief, etc.," from Old French adhérence, from Medieval Latin adhaerentia, abstract noun from Latin adhaerent-, present participle stem of adhaerare "stick to," from ad "to" (see ad-) + haerere "to stick" (see hesitation). Rare in a physical sense, adhesion being the usual word for that.

c. 1400, from Old French hesitacion or directly from Latin haesitationem (nominative haesitatio) "a hesitation, stammering," figuratively "irresolution, uncertainty," noun of action from past participle stem of haesitare "stick fast, remain fixed; stammer in speech," figuratively "hesitate, be irresolute, be at a loss, be undecided," frequentative of haerere (past participle haesus, first person perfect indicative haesi) "to adhere, stick, cling."

This is said by Watkins to be from PIE root *ghais- "to adhere, hesitate" (source also of Lithuanian gaišti "to delay, tarry, be slow"), but some linguists reject the proposed connection; de Vaan offers no etymology.

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.