Rule of Thumb

Wikipedia has an extensive discussion of this expression. It says it is “…a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination.” It is compared to heuristic, which is a similar concept used in math, psychology, and computer science.  The origin is uncertain, but likely comes several possible instances where the thumb is used to make an imprecise but convenient measurement. There is less evidence that the expression originated from prohibition of using a stick wider than the thumb to beat a wife.  Early colonial law often specifically rejected the concept. However, “…the phrase and connection gained currency in 1982, when the U.S. Commission on Civil Rights issued a report on wife abuse, titled ‘Under the Rule of Thumb’.”

On the Lam

William Safire wrote in the New York Times in 1998 that on the lam means “running away” or “being a fugitive from the law…” “The origin of the expression is in heated dispute among slang etymologists.” Allan Pinkerton, the first “private eye” said that pickpockets would say “lam” when they secured a wallet from a victim.” There is a Scandinavian verb lam that as early as 1525 was used to describe a wife “lamming him,” meaning to beat, pound, or strike” her husband. Mark Twain used the term twice to mean “to beat” in both instances. The connection is that “…to avoid a feared lamming (related to slamming), one lams. “

Pell-Mell

The Phrase Finder defines the term as meaning “In disorderly confusion; with reckless haste.” Another term used is to describe “…people charging about like chickens with their heads cut off.” Sir Thomas North translated “Plutarch’s Lives of the noble Grecians and Romanes,” a passage that indicated “disordered confusion.”  “He entered amongst them that fled their Campe pelmel, or hand overheade.” The expression is derived directly from the French. “Pel may derive from the Old French pesle, meaning to run or bolt.”

Off the Hook

I’ve noticed this expression has a new meaning lately.  Urban Dictionary notes it’s currently slang for something so new and fresh it is like the latest fashion right off the store’s shelf or hanger.  The meaning has expanded to anything cool or exciting.

Wiki.answers.com says the older meaning comes from fishing slang; if you’re on the hook you are caught, trapped, or obligated.  Off the hook means you escaped.  Phrase Finder has several definitions, including some no longer in use.  But I found no origin for the phrase.

Work Your Tail Off

I searched for an explanation of the origin of this expression, but found mostly explanations for the meaning. Several sites explained it referred to a person who worked diligently and usually with no obvious benefit to the worker. Ask.com had the most likely explanation for the origin, and it refers to the process of “docking” an animal’s tail. Docking is the intentional removal of the tail or sometimes ears to prepare the animal for some sort of practical function. One example was the “docking” of the tail of a horse to make it easier to use the horse for hauling. That makes sense, since the horse has part of its tail removed despite the fact the horse is working hard to achieve a work task after having part of their tails removed with no obvious value to the horse.

Cool as a Cucumber

Phrase Finder suggests this phrase plays on the two meanings of “cool” as a low temperature (cucumbers are cool to the touch) and imperturbable. The phrase was first recorded in a poem in 1732. The Online Etymology Dictionary adds that the idea that cucumbers feel cool was “confirmed by science in 1970: inside of a field cucumber on a warm day is 20 degrees cooler than the air temperature.” I am not surprised that a poet would create a memorable phrase, but for “cool as a cucumber” to last for nearly 300 years with its meaning intact is quite an accomplishment.