lengthyoudaoicibaDictgodict[length 词源字典]
length: [OE] Length was coined in the prehistoric Germanic period from the adjective *langgaz (source of English long) and the abstract noun suffix *-ithō. The resultant *langgithō has subsequently diversified to Dutch lengte, Swedish längd, Danish længde, and English length. (German has plumped for the different noun derivative länge ‘length’, whose English relative lenge ‘length’ survived until the 17th century.)
=> long[length etymology, length origin, 英语词源]
length (n.)youdaoicibaDictgodict
Old English lengðu "length," from Proto-Germanic *langitho, noun of quality from *langgaz (root of Old English lang; see long (adj.)) + *-itho, abstract noun suffix (see -th (2)).

Cognate with Old Norse lengd, Old Frisian lengethe, Dutch lengte. Figurative sense of "the distance one goes, extremity to which something is carried" is from 1690s. Phrase at length "to full extent" is attested from c. 1500.