Ask.Yahoo.com has several explanations for the origin, but writes that they prefer the explanation that spirits dwell in or guarded trees. The Greeks and Celts believed in tree spirits. Irish lore holds that “touching wood” thanks leprechauns for good luck. The Christian explanation is that the cross is the origin of good luck, “…although this is likely a Christian adaptation of earlier pagan practices.” A Jewish version is that persecuted Jews fleeing to synagogues built of wood to escape the Spanish Inquisition used a coded knock to gain admission. The practice saved lives and “…it became common to ‘knock on wood’ for good luck.”

The term is commonly used to describe left-handed pitchers. Yahoo Answers says there are reliable sources that the term was coined in Chicago where the stadium faced east and west with home plate on the west. Thus a left-handed pitcher threw from the south, and Finley Peter Dunne coined the term “Southpaw.”  Answerbag.com says it was common to build stadiums with home plate facing east so the late afternoon sun would not be in the hitter’s eyes.

Straight Dope says that some linguistic experts think that “spit” is derived from “spirit, “…noting that the southern pronunciation of the letter r is sometimes indistinct.” Thus the original expression could have been, “She’s the very spirit and image of her mother.” The intent is to say someone acts and looks like one of their parents.

O.K., so this isn’t an expression, but I found it interesting. The English.stackexchange.com says that some claim the origin was “…in the Middle Ages as a call-and-answer password format.” The only rule for the jokes seem to be that they be nonsense or at least silly. The Pittsburgh Press published several jokes in August of 1936 saying that a “new mid-summer game is going the rounds in place of monopoly.” One example given was, “Knock, knock! Who’s there? Landon. Landon who? Landon bridge is falling down.” Wikipedia reports that Fred Allen included a segment in his December 30, 1936 radio broadcast about “…the year’s least important events, including a supposed interview…” with “Ramrod Dank,” the man who was said to be the first to coin a knock, knock joke on the first of April. That is, of course “April Fools Day.”

The idiom is understood to mean that something will take an indefinitely long time based on cows being known to make their way at an unhurried pace. Phrase Finder says it appeared in print in Scotland in 1829. I enjoyed the reference to Groucho Marx saying to a female character in Duck Soup, I could dance with you till the cows come home. Better still, I’ll dance with the cows and you come home.”

There is no dispute that the expression means to pay an excessive amount for something. The origin is less clear. The Idioms Dictionary thinks it might come from the “nose tax” imposed in Ireland in the ninth century. Delinquent taxpayers were punished by having their noses slit. The Word Detective says it first appeared in the seventeenth century, and might equate being overcharged to being punched and given a nosebleed. “The theory is strengthened by the use of ‘bleed’ …to mean ‘cheat or defraud’.”