length (n.) Look up length at Dictionary.com
Old English lengðu "length," from Proto-Germanic *langitho, noun of quality from *langgaz (root of Old English lang; see long) + *-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.