along (prep.) Look up along at Dictionary.com
Old English andlang "entire, continuous; extended; all day long; alongside of," from and- "opposite, against" (from Proto-Germanic *andi-, *anda-, from PIE *anti "against," locative singular of *ant- "front, forehead;" see ante) + lang "long" (see long (adj.)). Sense extended to "through the whole length of."