People use idiomatic phrases all the time, but do they know where they’re from?  Every phrase has an origin, but people often do not know what it is.
“Happy as a clam” — This shorter phrase came from the larger phrase “as happy as a clam at high water,” which originated in the northeastern U.S. in the 18th or 19th century.  Some have suggested that clams have the appearance of smiling, but their “happiness” really had more to do with the tide.  Clams were easily gathered at low tide when they were exposed, but at “high water,” they were nearly impossible to find and therefore safe and “happy.”
“Cut to the chase” — This phrase is fairly recent; the first recorded instance of its use is from a 1929 novel about Hollywood by John McEvoy.  During this time, many films followed the same formula: the purpose of the main storyline was mainly to build up to the closing chase scene, which was the film’s main attraction.  In McEvoy’s novel, the phrase is used as a script direction.  To “cut” means to change scenes, so “cutting to the chase” would skip right to the action.
“It’s raining cats and dogs” — There is no consensus on this one, and supposed origins include mythology, archaic foreign words and phrases, and a fabricated tale of animals sliding off thatched roofs in the rain.  A more likely explanation, though not conclusive, comes from 18th-century England.  In 1710, poet Jonathan Swift described what may have been a common event at the time: dead animals, including cats and dogs, and other objects being washed down the streets of London during a heavy storm.

“Rule of thumb” — This phrase has unclear origins, but, contrary to some belief, is almost certainly not based on domestic violence.  It most likely stems from the use of one’s thumb for rough measurements, a practice that was common in woodworking.  In fact, most English measures of distance were based on body measurements, including the inch, which is the roughly the width of a man’s thumb.  However, similar phrases are found in other languages, including Persian and Swedish, suggesting that the phrase is very old and not unique to England.

    “Beat around the bush” — The most likely origin of this phrase comes from 15th-century hunting techniques.  To hunt boars, unarmed men would march around the forest, beating trees and making loud noises to scare them out from the bushes where they hid.  This way, they could avoid direct contact with the dangerous animals, in a similar manner to the way one avoids speaking about the often unpleasant subject at hand when “beating around the bush.”