The ear finger, digitus auricularis—i.e. the little finger. The four fingers are the index finger, the middle finger, the ring finger, and the ear finger. In French, le doigt auriculaire. The little finger is so called because it can, from its diminutive size, be most easily introduced into the conduit of the ear.
“Le doigt auriculaire est le petit doight, ainsi nommé parce qu'a cause de sa petitesse, il peut facilement être introduit dans le conduit auditif externe.” —Dict. des Sciences, etc.
The index finger. The first finger; so called because it is used as a pointer. The medical finger. The ring finger (q.v.).
“At last he put on her medical finger a pretty, handsome gold ring, whereinto was enchased a precious toadstone of Beausse.” —Rabelais: Pantagruel, iii. 17.
The ring finger. The finger between the long and little finger was used by the Romans as a ring-finger, from the belief that a nerve ran through it to the heart. Hence the Greeks and Romans used to call it the medical finger, and used it for stirring mixtures, under the notion that nothing noxious could touch it without its giving instant warning to the heart. It is still a very general notion in England that it is bad to rub on salve or scratch the skin with any but the ring finger. The fact that there was no such intimacy between the finger and the heart was not discovered till after the notion was deeply rooted. Pliny calls this digitus annularis.
With a wet finger. Easily. (See Wet Finger.) My little finger told me that. The same as “A little bird told me that,” meaning, I know it, though you did not expect it. The former expression is from Moliée's Malade Imaginaire. (See Bird.)
By the pricking of my thumbs, Something wicked this way comes.
Shakespeare: Macbeth, iv. l.
Cry, baby, cry; put your finger in your eye, etc. This nursery rhyme seem to be referred to by Shakespeare in his Comedy of Errors, ii. 2:
No longer will I be fool, To put the finger in the eye and weep.
To hold up a finger (in an auction room) by way of a bid, was a Roman custom, “digitum tollere” (Cicero: In Verrem, Actio i. 54). Horace confirms this.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894