The finger a girl uses to masturbate with. plural form would be magic fingers.
Mary: yeah i was masturbating last night because i was just so wet
Dave: oh, so which finger is your magic finger?
Mary: Left hand, middle finger.
When performing sexual intercourse, a man is turned on so much, he feels confident enough to shove his finger into his partners anus, causing much pleasure.
"Dude Jigar gave me the magic finger last night when we were doin it."
"Sweet, can I try?"