jargon.artrocity.com
New Hacker's Dictionary the Jargon File v 4.4.6
[Usenet] The finger, in the sense of digitus impudicus. This comes from an analogy between binary and the hand, i.e. 1=00001=thumb, 2=00010=index finger, 3=00011=index and thumb, 4=00100. Considered silly. Prob. from humorous derivative of finger, sense 4.