With a grain of salt: To take something "with a grain of salt" is to have a healthy dose of skepticism or suspicion. Salt was once believed to have healing properties and to be an antidote to poison. To eat or drink something with a grain of salt was to practice preventive medicine against potential poisoning or illness. Because so much possibly apocryphal information is out there, it would be wise to read all of these explanations with that "grain of salt."
High on the hog: Living "high on the hog" means living extravagantly. It originally meant eating what were considered the superior cuts of meat -- the ones on the higher part of the animal, like pork chops, hams, etc. Less well-off people could afford only the lower cuts -- the belly, feet, chitterlings, etc.
The whole hog: Another porcine reference, it means to go the limit -- all the way. The likely origin of this phrase is William Cowper's 1779 poem, "The Love of the World Reproved." It tells the story of a group of religious, but starving, Mohammedans who, after being ordered by the Prophet not to eat a certain, unspecified part of a pig, end up eating the entire animal when they can't determine which part is forbidden.
Pie in the sky: This term for an unachievable dream comes from an early 20th century folk song by labor activist Joe Hill. It's a satiric attack on the Salvation Army, whose preachers, Hill felt, lulled workers into complacency. The first verse of "The Preacher and the Slave" goes:
You will eat, bye and bye,
In that glorious land above the sky;
Work and pray, live on hay
You'll get pie in the sky when you die.
Chew the fat: This means to chat or gossip idly. The Inuit used to chew on pieces of whale blubber almost like chewing gum. Because the blubber took a while to dissolve, chewing helped pass the time while doing something else. Another theory is that this expression has nautical roots. When food supplies were low, sailors had to chew on salt pork, and they griped about the poor food as they chewed. Two very appetizing images, aren't they?
Happy as a clam: The phrase was originally "happy as a clam at high tide." It seems that clam diggers are able to catch clams only at low tide. So when the tide is high, the water is too deep to wade into and the clams were safe from the diggers.
I researched lots of other phrases, but space limitations prevent me from including them here. That's the way the cookie crumbles.