assure 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.

1530s, "without care or fear, dreading no evil" (a sense now archaic), from Latin securus, of persons, "free from care, quiet, easy," also in a bad sense, "careless, reckless;" of things, "tranquil; free from danger, safe," from *se cura, from se "free from" (see se-) + cura "care" (see cure (n.)).

In early use it often implied "over-confident, too sure." In English, in reference to places, "free from danger, unexposed," by c. 1600. The mechanical meaning "firmly fixed" (of material things) is by 1841, extended from the mental meaning "affording grounds for confidence" (1580s) hence "of such stability, strength, etc. to preclude risk." Of telephones or telephone lines, "not wiretapped," by 1961.

The earlier word, or form of the word, was Middle English siker, from Old English sicor, an earlier borrowing of the same Latin word, and sure (adj.) is a doublet, altered in its passage through Old French. Related: Securely.