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a prefix occurring in loanwords from Latin, where it meant “toward” and indicated direction, tendency, or addition: adjoin . Usually assimilated to the following consonant; see a-5, ac-, af-, ag-, al-, an-2, ap-1, ar-, as-, at-.

Origin:
< Latin ad, ad- (preposition and prefix) to, toward, at, about; cognate with at1

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ad-
 
prefix
1.  to; towards: adsorb; adverb
2.  near; next to: adrenal
 
[from Latin: to, towards. As a prefix in words of Latin origin, ad- became ac-, af-, ag-, al-, an-, acq-, ar-, as-, and at- before c, f, g, l, n, q, r, s, and t, and became a- before gn, sc, sp, st]

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Word Origin & History

ad-
prefix expressing direction toward or in addition to, from L. ad "to, toward," from PIE *ad- "to, near, at" (cognate with O.E. æt; see 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 (e.g. affection, aggression). In O.Fr., reduced to a- in all cases, but written forms were refashioned after L. in 14c. in Fr., and 15c. in Eng. words picked up from O.Fr. In many cases pronunciation followed the shift.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

ad- pref.

  1. or ac- or af- or ag- or al- or ap- or as- or at- Toward; to. Before c, f, g, k, l, p, q, s, and t, ad- is usually assimilated to ac-, af-, ag-, ac-, al-, ap-, ac-, as-, and at-, respectively: adductor, acclimation, agglutinant.

  2. Near; at: adrenal.

The American Heritage® Stedman's Medical Dictionary
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