finger

   1. to inform on or point out in a criminal context

   The pointing is usually figurative only:

    Snyder had hoped to pick up a few hundred bucks by fingering Hooker to Amon Lorrimer. (Weverka, 1973)

   To put the finger on is also to betray. The betrayer is a finger-man.

   2. to point out an opportunity for a crime

   This time the pointing out is to another criminal:

    I figure he knew them, and they knew him. Maybe he fingered the job. (Sanders, 1977)

   Again the agent is a finger-man or finger guy:

    The finger guy must know the party he fingers has plenty of scratch to begin with. (Runyon, 1990, written in 1930s).

   3. to excite (another) sexually with a finger

   Usually of a female by a male:

    There was a young fellow of Bude

   Who fingered his girl while they queued...

   (vulgar limerick)

   To finger (yourself) is to masturbate, of male or female:

    ... her other hand fingering, all five fingers fingering like a team of maggots at her open heat. (Mailer, 1965)

    Bouts of screaming followed and then, as though in a trance, she had begun to finger herself. (Fiennes, 1996)