Is it ensure, insure, or assure?
Old English andlang "from end to end, lengthwise" (see along) with Middle English substitution of ende (see end (n.)) for first element. Meaning "at full length, horizontally" is from early 15c. In Middle English frequently paired with overthwart and together meaning "lengthwise and crosswise," hence "in all directions." As a preposition c.1200. see along.