The prep. and in comb., Goth. anda-, and-, OHG. ant-, ent-, ‘against, in return, opposite, fronting, toward.’ In OE. the full form (often labialized to ǫnd-) remained only under the stress, i.e. in ns. and adjs.; as ˈand-git apprehension, ˈandsǽte opposed, hostile, ˈandswaru answer, ˈandweard present. When proclitic, as in vbs., it was weakened to ǫn- as ǫnˈgitan to apprehend. The former still remains as an- in answer (see an- prefix 2); the latter, like ǫn- for an-, afterwards fell to a-, as in along (see a- prefix 4).
Useful english dictionary. 2012.