finger (n.) Look up finger at Dictionary.com
Old English fingor, from Proto-Germanic *fingraz (cognates: Old Saxon fingar, Old Frisian finger, Old Norse fingr, Dutch vinger, German Finger, Gothic figgrs), with no cognates outside Germanic; perhaps connected with PIE *penkwe-, the root meaning "five."

As a unit of measure (Middle English) it represents the breadth of a finger, about three-quarters of an inch. They generally are numbered from the thumb, and named index finger, fool's finger, leech- or physic-finger, and ear-finger.
finger (v.) Look up finger at Dictionary.com
early 15c., "to touch or point to with the finger" (but see fingering from late 14c.), from finger (n.). Sense of "play upon a musical instrument" is from 1510s. The meaning "identify a criminal" is underworld slang first recorded 1930. Related: Fingered; fingering.