Dear Straight Dope:
While trying to find an answer to this question, the search engines continually suggested your archive pages. I couldn't find a discussion of it, though. It was returning your pages because of the last line on the pages:
Cecil's Mailbag is researched and written by members of the Straight Dope Science Advisory Board, Cecil's online auxiliary. Although the SDSAB does its best, these articles are edited by Ed Zotti, not Cecil, so accuracy wise you'd better keep your fingers crossed.
Do you know the origin of the gesture?
SDStaff Dex replies:
Ah, the great advantage (and disadvantage) of web searches, eh? Well, just so you didn’t waste your time …
Cecil didn’t want to bother with this one, because it’s been done. Charles Panati, in Extraordinary Origins of Everyday Things, has a nice article on crossing one’s fingers as a sign of luck or making a wish. He traces it back to pre-Christian times, when the cross was a symbol of unity and benign spirits dwelt at the intersection point. A wish made on a cross was a way of “anchoring” the wish at the intersection of the cross until the wish was fulfilled.
Panati says this superstition was popular among many early European cultures. It originally took two people. A comrade or well-wisher placing his index finger over the index finger of the person making the wish, the two fingers forming a cross. The one person makes the wish, the other empathizes and supports. Over centuries, the custom was simplified, so that a person could wish on his own, by crossing his index and middle fingers to form an X. But traces remain — two people hooking index fingers as a sign of greeting or agreement is still common in some circles today.
Panati comments, “Customs once formal, religious, and ritualistic have a way of evolving with time to become informal, secular, and commonplace.” Thus, friends crossing fingers evolved (Panati says “degenerated”) to crossing one’s own fingers, and ultimately to the stock phrase, “Keep your fingers crossed,” with no actual finger-crossing at all.
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STAFF REPORTS ARE WRITTEN BY THE STRAIGHT DOPE SCIENCE ADVISORY BOARD, CECIL'S ONLINE AUXILIARY. THOUGH THE SDSAB DOES ITS BEST, THESE COLUMNS ARE EDITED BY ED ZOTTI, NOT CECIL, SO ACCURACYWISE YOU'D BETTER KEEP YOUR FINGERS CROSSED.