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cubit (n.)[cubit 词源字典]: ancient unit of measure based on the forearm from elbow to fingertip, usually from 18 to 22 inches, early 14c., from Latin cubitum "the elbow," from PIE *keu(b)- "to bend." Such a measure, known by a word meaning "forearm" or the like, was known to many peoples (Greek pekhys, Hebrew ammah, English ell).[cubit etymology, cubit origin, 英语词源]
ell (n.1): unit of measure, Old English eln, originally "forearm, length of the arm" (as a measure, anywhere from a foot and a half to two feet), from PIE *el- (1) "elbow, forearm" (cognates: Greek olene "elbow," Latin ulna, Armenian uln "shoulder," Sanskrit anih "part of the leg above the knee," Lithuanian alkune "elbow").

The exact distance varied, in part depending on whose arm was used as the base and whether it was measured from the shoulder to the fingertip or the wrist: the Scottish ell was 37.2 inches, the Flemish 27 inches. Latin ulna also was a unit of linear measure, and compare cubit. The modern English unit of 45 inches seems to have been set in Tudor times.
Whereas shee tooke an inche of liberty before, tooke an ell afterwardes [Humfrey Gifford, "A Posie of Gilloflowers," 1580].
finger-tip (n.): also fingertip, 1817, from finger (n.) + tip (n.). Related: Fingertips. To have something at one's fingertips is from 1870.