(Latin for grain of salt) The idea for grain of salt as a remedy first appeared in Pliny’s Naturalis Historia, 77 A.D. and it translates this way: After the defeat of that mighty monarch, Mithridates, Gnaeus Pompeius found in his private cabinet a recipe for an antidote in his own handwriting; it was to the following effect: Take two dried walnuts, two figs, and twenty leaves of rue; pound them all together, with the addition of a grain of salt; if a person takes this mixture fasting, he will be proof against all poisons for that day. It was much later that the term took on a warning of sorts for things said/heard.  You can read about it here . 

Especially important in Asian cultures is the idea of saving face. You can read about the origin of this phrase here . The phrase means to protect, shelter, or save someone's reputation.

In the late 1800s when a baseball game was rained out the attendees were given the option of a rain check which would allow them to come to a different game. So the term rain check is very literal in its origin.  You can read more about it here as well as a couple of other ideas about the term. "Can I take a raincheck?"  Today it's a term used to politely decline an invitation. 

Apparently the English hate the French and that's where this term comes from. It's meant as an insult toward the French, insinuating that they are dirty, etc.  Here's one article that gives some great background on the immense dislike the English have for the French.  When an English person would do or say something crude the response would usually be along the lines of "pardon, are you French?" Here's another explanation for the history behind this term.  And one of the most well known uses of the phrase...

One idea of where this phrase comes from is the telephone and its handset. Click here for that explanation.  Another possibility is found in fishing. It's much more literal. Just as a fish released off the hook is released from the obligation of being someone's meal so is a person who has been let loose from a responsibility, they are off the hook. 

The origin of this phrase is unknown. It was first seen in print in the 18th century when political writer Edmund Burke said “having looked to government for bread, on the first scarcity they will turn and bite the hand that fed them.” ( source ) Based on that, and the definition which includes the idea that biting the hand that feeds you is a way to say disrespect is being shown to the person providing necessities, the following are some examples of the phrase.

One explanation. which is the most common one, dates from the early 1800s and alludes to exposure to the chemicals formerly used in making felt hats, which caused tremors and other nervous symptoms.  (source)    There is another explanation and it's the one I think makes more sense given the picture above. The variant, dating from the 16th century, alludes to the crazy behavior of hares during rutting season, mistakenly thought to be only in March. (source) Of course, when Mad as a Hatter or Mad Hatter is uttered today most everyone thinks of The Mad Hatter from Lewis Carroll's Alice in Wonderland . And the character does exemplify what the modern use of the phrase is today - someone who is crazy whether in anger or in behavior.