alongyoudaoicibaDictYouDict[along 词源字典]
along: [OE] The a- in along is related to the prefix anti-, and the original notion contained in the word is of ‘extending a long way in the opposite direction’. This was the force of Old English andlang, a compound formed from and- ‘against, facing’ (whose original source was Greek anti- ‘against’) and lang ‘long’. The meaning gradually changed via simply ‘extending a long way’, through ‘continuous’ and ‘the whole length of something’ to ‘lengthwise’.

At the same time the and- prefix was gradually losing its identity: by the 10th century the forms anlong and onlong were becoming established, and the 14th century saw the beginnings of modern English along. But there is another along entirely, nowadays dialectal. Used in the phrase along of ‘with’ (as in ‘Come along o’me!’), it derives from Old English gelong ‘pertaining, dependent’.

This was a compound formed from the prefix ge-, suggesting suitability, and long, of which the notions of ‘pertaining’ and ‘appropriateness’ are preserved in modern English belong.
=> long[along etymology, along origin, 英语词源]
along (prep.)youdaoicibaDictYouDict
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."