Askville.Amazon.com says the expression means trying to remain cheerful in difficult circumstances.  “It is a great temptation to try to cheer oneself up by whistling or singing in a dark and lonely place” One reference can be found in Robert Blair’s “The Grave” from 1742. “The Schoolboy…Whistling aloud to bear his Courage up.” A comment said that villages were a day’s travel apart in the Middle Ages, cemeteries were at the edge of towns, and spirits came out at dusk to frighten travelers. The travelers believed loud noises would keep the spirits away. Some would bang on pots and pans and others would whistle or shout.

A family member suggested this expression, and the explanation of its meaning and origin on Wikipedia (under line in the sand) is interesting. The first meaning is a point “…beyond which one will proceed no further.” The second is it is a point at which a decision is permanent and irreversible. There is a Biblical link in John 8:6 where “…Jesus draws a line in the sand to address those who are about to stone a woman caught in adultery. There are several references to “…historic, or legendary, military events…”  (including the Alamo) in which a version of the expression was said to have been used.

The expression, which apparently had the original spelling “hookey,” is commonly used to express missing some scheduled event without permission. Mark Twain used it to describe how Tom Sawyer “took his flogging, along with Joe Harper, for playing hookey the day before.” The Urban Dictionary says the term is probably derived from the Dutch term “hoekje” for the game of hide and seek based on the word “hoek,” which means corner. The expression evolved into playing hooky to describe children skipping school. There is an alternative origin based on the verb hook meaning “to steal.” However, “hook it” didn’t make it into common use until playing hooky was already in use.

Wordwizard.com has several explanations from different sources for this expression, which is used to describe defeating someone decisively. As William Safire explained in the New York Times, the phrase begins with “clean,” which was applied to the victims of thieves or gamblers who had been drubbed, defeated, and/or wiped out. The “clock” probably comes from the fact a clock has a face, and “clock him one,” refers to hitting him in the head. Brewer’s Dictionary of Modern Phrases & Fable has a similar explanation saying that the expression has a military origin from World War II, and that, “A person’s face is probably their face…” Cassell’s Dictionary of Slang speculates it might be railroad jargon referring to applying the airbrakes and bringing the train to a sudden stop. “The ‘clock” in question is the air gauge, which on halting, immediately registers zero and is thus ‘clean’.”

This expression has become popular to warn of the less obvious results or effects of a situation or event. It is based on the fact that only ten percent of an iceberg is above the surface of the ocean. According to the Merriam Webster dictionary, the expression became popular beginning in 1969. In my personal opinion, “Tip of the iceberg” could be used to predict unintended consequences that often show up when the government tries to solve a problem with complicated laws.

World Wide Words says the expression is used to describe an “awkward state of affairs.” The origin of the idiom is uncertain, but it is known that gentry on the Scottish border with England would feed guests at outdoor outings by boiling salmon in kettles over open fires. There is a guess that the fish broke up in the process of being boiled and “looked messy” (yum!). Another guess is that the expression results from the corruption of a quintal or kentle (one hundred pounds) of fish. I prefer the first guess, since it produces such a messy image.