along (adv., prep.) Look up along at Dictionary.com
Old English andlang "entire, continuous; extended" (adj.); "alongside of" (prep.), 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.)). Reinforced by Old Norse cognate endlang. Prepositional sense extended in Old English to "through the whole length of." Of position, "lengthwise," c. 1200; of movement, "onward," c. 1300. Meaning "in company, together" is from 1580s. All along "throughout" is from 1690s.