Quarantine

Quarantine - A condition of enforced isolation.

The word comes from a Venetian custom in the Middle Ages when ships arriving from the reported plague-stricken countries were obliged to spend 40 days (Italian "quaranta" or "forty" comes from the Latin "quadraginta," also meaning 40) at the port, in isolation, before being allowed to unload its cargo and crew. Venice, in those days, was the chief European port of entry and Europe had experienced many epiemics of plague. Forty days was supposed to be long enough to kill the infection for goods (and people) by exposure to air and sunlight.
The current usage of the word is not limited to 40 days but "any period" of isolation.

Trivia: Quarantine first appeared as a legal term in 1609, as the period of 40 days in which a widow could remain in her dead husband's house before creditors could seize it.

Sources: answers.com, etymonline.com, Wikipedia

Mad as a hatter

Mad as a hatter - Crazy, demented

Reminds you of Alice's companion in her wonderland? I used to think the expression was Lewis Carroll's gift to the language just as jabberwocky is. Turns out the phrase was popular well before Carroll's "Alice in Wonderland" was published. "Mad hatter syndrome" was actually a medical affliction in Carroll's times.In the mid-1800s, hat makers used hot solutions of mercuric nitrate to shape wool felt hats and prolonged exposure to mercury vapors caused severe neurological damage ranging from uncontrollable muscular twitching (known as "hatter's shakes") to dementia. Hatters working in poorly ventilated workshops would breathe in (elemental) mercury vapor and in advanced cases, developed hallucinations and other psychotic symptoms.

Source: http://www.word-detective.com
Pic : www.rjohnwright.com

The Seven-year Itch

The inclination to become unfaithful after seven years of marriage.

Remember Marilyn Monroe's dress blowing in the wind over a subway grating? One of the most enduring images from Hollywood, the scene was from the movie, The seven-year itch. The expression was used to indicate the urge for infidelity after seven years of marriage. Though today it has gained wider acceptance in terms of its scope: it now refers to an urge to move on from any existing situation, and not even limited to those of seven years' period.

The original seven-year itch wasn't a condition that supposedly began after seven years, but one that supposedly lasted for seven years. Seven-year itch had been known in the USA since the early 19th century as the name of a particularly irritating and contagious skin complaint (don't ask me which one, coz despite thorough search I could not find the answer to that: it could be scabies or poison-ivy itch (very unlikely though) ) that led to highly irritating red pimples on the face and body.

The term was virtually forgotten after the cure for the complaint was found till the 1955 film revived it.

Source: http://en.wikipedia.org/ , http://www.phrases.org.uk/,

Update: (05/05/07) by 666

The follow-up research to Moi’s post was incredibly fascinating. I came across numerous citations (mentioned below) explaining some sort of link between ‘seven year itch’ and scabies.

The 'seven year itch' has its origins with a microbe known as Sarcoptes scabiei, more commonly called 'scabies.' The bug produces an itching skin irritation that before modern drugs lasted, on average seven years.
"Why seven years, not six or eight? Because seven years has a historical basis: In Genesis, Joseph interprets Pharaoh's dream of 'seven years of great plenty' followed by 'seven years of famine.'." And so forth. – (1)

However, the medico’s have a slightly different version. Scabies is known as "The Seven Year Itch," because its incidence rises and falls as regularly as the tides, or the sunspots, turning up in abundance every seven years. – (2)

Read in detail about scabies here and here.

Sources
(1) - http://www.phrases.org.uk
(2) - http://www.ralphmag.org/DI/scabies1.html