finger (n.)youdaoicibaDictYouDict[finger 词源字典]
"terminal or digital member of the hand" (in a restricted sense not including the thumb), Old English finger, fingor "finger," from Proto-Germanic *fingraz (cognates: Old Saxon fingar, Old Frisian finger, Old Norse fingr, Dutch vinger, German Finger, Gothic figgrs "finger"), with no cognates outside Germanic; perhaps connected with PIE *penkwe-, the root meaning "five."

As a unit of measure for liquor and gunshot (late Old English) it represents the breadth of a finger, about three-quarters of an inch. They generally are numbered from the thumb outward, and named index finger, fool's finger, leech- or physic-finger, and ear-finger.[finger etymology, finger origin, 英语词源]
teacher (n.)youdaoicibaDictYouDict
"one who teaches," c. 1300; agent noun from teach (v.). It was used earlier in a sense of "index finger" (late 13c.). Teacher's pet attested from 1856.