digit (n.)

late 14c., "numeral below 10," from Latin digitus "finger or toe" (also with secondary meanings dealing in counting and numerals), considered to be related to dicere "to say, speak," from PIE root *deik- "to show," also "pronounce solemnly." Numerical sense is because numerals under 10 were counted on fingers. The "finger or toe" sense in English is attested from 1640s.